I use scissors, and I will admit to having the odd mistake in perhaps cutting a mm or two of image with 35mm film. A small number of mistakes in 36 years of film developing isn't bad. I find that as I get older, my ability to focus close up isn't what it was. Plus a lot of my photography now is gigs shot in a dim jazz club so the frame edges aren't always easy to see. 120, 127 and 110 (I do them all too) are no problem.
If and when I have the space to build a proper dark room (or if I am successful in persuading the local arts centre to install one) I will either go the light box route or get a small guillotine type device I think.
One thing is I use sharp craft scissors, one thing the wife doesn't mind me borrowing from her craft supplies. She probably cuts much thicker materials with them.
35mm I cut into strips of anything between 4 and 6 and store in those sheets that fit into ring binders. Six is the max per row, and is my usual number....I'll do a shorter strip at the end of a film or if there are 8 negatives left after I've done a few rows of 6 I'll do the final two rows of 4. I don't really like doing less than 4. For 120 it's rows of three 6x6 or two 6x9 in similar sheets.