Whenever possible, trying using distilled or rain water. Dracaenas are sensitive to chemicals in tap water.
Agree, adding rocks at the bottom of the pot doesn't help with anything, except to make it heavier. Does the pot have a hole in the bottom?
If you want new foliage to start forming about 2 feet up, that's a good place to cut. A clump appearance works best, IMVHO, when the foliage clumps are at different heights. I would encourage you to wait until next summer to do the cutting if possible, when longer days and warmer temps can aid in the recovery of the stump and propagation of the removed top.
