Mine never slipped, but I saw the stepper motors moving around as the lead screw turned due to the rigid connection and thought that wouldn't be good over the long term. One of my earlier mods was replacing all of my motor couplers with these, and I've never had a problem with them loosening up:http://www.amazon.com/gp/product/B00GSMSPKQ?psc=1
My lead screw is 3/8" (9.525mm) and I think it's pretty standard that NEMA 23 steppers have 1/4" (6.35mm) shafts. I don't know which generation of V90 you have or if it changed between V90 and X90, probably not, but worth checking.
When installing, I like to leave a little gap between the stepper and lead screw, so they don't rub against each other within the coupler. In fact, I like the lead screw to go no further than the clamp portion itself. I don't think the cavity within the flexible area is any bigger than the lead screw itself. Either can defeat most or all of the point of using these couplers.
Make sure your shaft lock collars are tight against the bearings. Since these couplers are essentially springs, they won't do anything to stop backlash in the lead screw if the lock collars are loose.