Peronal experience with spuetering, is USUALLY they do fine without the collars. (Oh, I've had many kitties in my life!) I had one friend (none of mine) whose girl ripped out her sticthes the first day, and off to the ER vet! But even my last girl who was NOT happy about being spayed, never did anything to the stitches. (Rats tend to get the pica/weird chewing thing going on...kitties are more tolerant).
My one boy kitty, who we fondly at 5 years called "transgendered kitty" had to have a surgery for urine crystals that was...really kind of a transgender surgery! (They removed and rearranged his male parts!) He did wear a cone kind of thing for a few days.
I've seen many kitties, male and female spueterd and USUALLY it works out fine.
I would just personally say, if they tolerate the collars for a few days, great. I really wouldn't worry too much about the boys. If they can't stand them, even the girls, not too much worry.
I think you are going to a clinic where they will send them home sooner than later, so if it were me, I'd let the collars stay on for at least a day.
Just know that if they do chew out their stitches, you'll have emergency vet funds. But in all of my years, all of my kitties, no one has.
PS: Don't be surprised if they aren't terribly happy about it! I use a ramp of some sort to their favorite place to sleep so they don't have to jump.