I wish to have you research out this idea before you do it. It is a common belief that putting rocks in the bottom of the container helps with drainage. In actuality, this is false, the rocks actually manage to keep that area wet down there and really don't help with the drainage of the soil at all.
Soil has a dynamic that is built into the mix, called its flow through ability. This is the ability for water, because of gravity, to flow through the soil from top to bottom and if there is too much for the soil to be able to hold (the water retention rate) it will flow right out of the soil and down through the drainage holes. Soil should need no help draining, if it is well designed. Compare a typical soil to a clay, to visualize the difference in flow through rate. Clay might be able to hold a lot of water, but it doesn't go anywhere.
The other problem with the rocks in the bottom is the amount of extra surface area with all the new structures down there. Because of capillary action between the sides and the bottom and those rocks, that area remains wet long after the soil above has dried out. This area is ripe for the cultivation of bad microbes... no oxygen, dark and stagnant water... it is a recipe for failure. Find a planter that has used this method, and lift the mess out of there so you can smell what is down there. That stink... that nasty smell, is bad microbes, algae and other nasties. I recommend putting a thin layer of aged compost down there... not too much or you stop the flow through... but enough to give the roots something to enjoy when they find it down there. Other than that, put that soil all the way down to the bottom and let the flow through do its thing.