The problem as I see it is that we have become victims of our own popularity. Back in the day there were not many cannabis websites or books and the brave pioneers in this hobby used standard gardening techniques and old school methods. Nowadays, every dispensary has a website and grow experts abound... but are they really experts? I can name several websites and facebook pages that regularly offer horrible advice to their growers, labeling anything they can't accurately diagnose as the dreaded Tobacco Mosaic Virus, and giving bad advice about flushing.
First of all, in the last 5 years the definition of flushing has changed... a frightening percentage of new growers have no idea what it really means to flush your soil. And now, with so much confusion about flushing, many people have decided not to do it, or as was just posted, that there is no right answer to the question.
Of course there is a right answer and as the state of the art progresses, we are even wiser today than we were a decade ago about this practice, but even so, because of the proliferation of bad advice sites all over the internet, many people are still in the dark about this practice.
It used to be the quick answer on most online forums when presented with a plant problem, to flush. That has always been a problem, those with no valid advice offering this panacea to solve all problems, and oftentimes it actually worked, but as we know today it isn't always the best answer.
So first of all, what is a flush? Is it the act of giving pH adjusted water? No, that is simply watering with no nutes, it is not flushing. How about giving fluids to 20% runoff each time, cleansing the soil a little bit each time... is that flushing? Again, no... If you have a pile of salt sitting in your soil that keeps accumulating as you give more nutes, and you deplete that pile by 10% every time you water, but you in the process add back 5% with the new nutes, how much have you gained? Not much. If that pile of salt gets to the point that its starts locking out nutrients, does the incremental "flush" solve the problem? Nope.
The only true flush is to run 3x the container size in pure water through the system, dragging out the salt and debris with it. Some people even use sugar water to help dissolve the salt, so that maybe they don't have to run quite that much water through, but the goal is to clean out the soil. 10% wont do that... nor will 20%... to properly clean that medium out it takes 300%.
Do we flush our plants? No... what a silly idea this is. The notion of cleansing the plant of all nutrients before the end, giving only water for a week or (egads) two weeks, does not clean all the collected nutrients out of the plant. It does make the plant think it is dying, so it hastens the end and looks all impressive and all, but what did you sacrifice to get there? You starved the plant of nutrients, important finishing nutrients, right at the end when the plant needed that nutrition the most.
On hearing this, people defend their actions saying that their plants were just fine, great in fact, following this practice... but you can't prove something that didn't happen. These growers might be satisfied with what they ended up with, but they could have no idea to what potential that plant may have reached if it had not been starved that last 2 weeks.
Recent science has proven that water only at the end does not take the nutrients out of the plant. As stated in this thread, people who try a true 3x flush at the very end, or just water at the last week or two, can NOT taste the difference in their product. This is because we now know that neither practice affects the taste, smell, burnability or anything else about the cured product.
Old school taught us to flush at the week or 2 weeks from the end point... a true 3x flush. Why then? And then what, water only the rest of the way, or regular nutes? Here are the old school answers. I don't care what this website or that says about this; you can't always believe everything you read on the internet.
A true flush is to remove salt, debris or excess nutes from the soil... not the plant. Flushes are recommended when there is a major nute change, such as going from veg to flower, or from budset to budbuilding. Flushes are also to remove the salt that builds up in the soil as nutes break down to be used by the plants. After several weeks of heavy synthetic nutes, appreciable amounts of salt build up in the soil, so much so that it begins to restrict water uptake into the roots. Too much salt and you can actually lock out nutrients and the entire metabolism of the plant will slow down. Periodically, it is wise to clear out those salts so that your roots can work unrestricted and uptake the maximum amount of water and nutes that they are capable of.
There is no more important time to do an actual flush than that point at a week or 2 weeks before the end. The buds can swell up to twice their size and weight in that last two weeks, but in a salt restricted plant instead of rapid building of the buds at this point, a slow down in water usage and growth is seen. Doing a true flush at this point opens the floodgates, and allows the plant to draw up into the roots everything that it wants, and the difference that this final flush can make is astounding. For years and years this was common knowledge... but now people seem to have forgotten many of the old ways, and again, I blame the internet.
So after you do this final flush 2 weeks till the end, what then? You are still waiting for the final budswell to finish, the pistils to pull back and the trichomes to turn to your desired point. Water only you say? My answer is, are you daft? You give nutes to make big plants up to this point, and then you go water only at the most critical time??? Please. Use your head. This is not how to grow the best pot you can. Since we now know that the nutes do not show up in the flavor of the final product, what is stopping you from feeding the plant just as you have been doing, right up to the end? Nothing but this odd modern belief in bro science... if it comes off of youtube, it must be correct.
Sometimes, the old ways are the best ways.
Alright, I have had my say. Have a great evening everyone!
The bonus to that of course is that it saves a bunch of trimming ha ha- so I was happy about that. I admit I always really love being able to chop a plant in 10 minutes... I was looking forward to that 
