What strain is it hun?
Some are really touchy about being topped or tied - the breeder info usually will say so if it is an issue.
Topping is a matter of preference--
the longer it grows before topping, the further up the plant it will branch...
sometimes too high-up will make the fork of the plant the weakest point, and when buds get big, the plant can split there-- so you cut it lower and use the cut-off portion as a clone (bonus!)
Often, people "FIM" (F**k I Missed) by pinching out the new growth at the very top of the plant and letting it go from there-- but it appears your plants are too tall for that now-- but you could do that to the "clone" you cut from it....
IMO- cut the entire top off- right at about the height of the base of the neck of that bottle, as it appears to already be a bit streatched between those points, and that top is definately big enough for a clone....
again-- JMO, and you should check the breeder info before ya cut her!!!
Good Luck! looking good!