if you're doing hydro... empty the res and give them 2-3 days of dry-ness
if you're doing soil/soil-less medium... I stop my waterings about 3 days before I chop
you want the plant(s) to use up the excess moisture that they've stored in their stem and fan leaves... but you don't wanna wait so long that your plant begins to droop. I waited too long once, the plant began to droop badly, and it took almost 3 times longer to trim.
hope this helps