Depends on the Brand as well.
NRS didnt recommend trimming, just stretching, (last i checked 5 yrs ago....am long since done with NRS dry wear, poor quality gaskets.Lots of other great gear from them, but skip the drywear...)
Kokatat recommends stretching wrists gaskets and trimming the neck.
carefully, of course. as everone else said.
I just got my dysuit back from koko with new gaskets, (awesome repair dept,3 week turnaround...)
and I had to do this myself. I was nervous about it too, but its not bad if you dont stretch the gaskets too tight.(too much tension stretched around too wide of an object will cause bad results when you cut it... )
i left it stretched for a couple weeks first, but there was no way (i have a thick ol' neck as well)
I used a windshield washer fluid gallon jug for the neck and constuction rolls for the wrists to stretch on. PB jars would work maybe.
i would recommend first, check kokatat.com, care instuctions, and google it. check the youtube dIY videos that are available,a couple different techniques are always good reference.
use a scalpel or razor, and a plastic cookie jar (i actually found one from Xmas that worked great, unlikely as it may be, or the 1 gal. paint bucket would be perfect..) turn the gasket inside out, and cut just above the rings, in a long smooth cut, to avoid making the little jagged edges that will shred your neck for the next 4 years.
if you use a good sharp razorblade and have good light, its not too bad. i cut 2 rings at first, and then one more cause it was still too tight. pull the cut ring away from the rest of the gasket to help it separate, to avoid multiple passes. and jagged cuts.
the dude in the video has good pointers, dont rush it, make smooth cuts, and dont cut too much cause you cant take it back. to test it, wear it for 20 minutes at home to see if you can tolerate the squeeze. mines still tight, cut down 3 rings, but once hopefully, it will get better sooner then later.. just need to actually get out and stretch it the right way...in cold water...lol
hope this helped a bit. good luck!