Well, to me it depends on the amount of light. If you have 75-100 watts per sq.ft. then you can let them get bigger, like 1 foot with multiple tops. If you are using CFL's or Flouro's or have 50w or less per sq.ft. then I would flower at approx. 10" of height with multiple tops.
To get the multiple tops you gotta top (cut) or tie down (train) the plant.. .and now's the time to do that. Look up the LST thread on training or if you're lazy just tie down the top half of the plant until the top is lower than some of the branches so they will develop more.

Good luck!