I make up a hydro solution, and let it go for a week, yes, but I check pH daily if I'm around. if pH is off, I check EC. If EC is off, I correct. otherwise, I try not to fiddle with it too much. Remember EC will not tell you what is in there, only that it is in there to a certain concentration.
Retopping with nutes back up to a certain level, CAN, (not will) cause a toxicity in nutrients if the plants are not taking everything equally. And that they rarely do, but it is possible to harm your plants by doing this, so I just choose to change out every 2 weeks, or if they drink it all in a week, I change then.