I now stay away from them tools that cut the sides of the tips to get it flush. I've nicked plenty of ferrules with them. I would rather go slow with a razor blade and do it by hand.
I do the initial cutting with tape around the ferrule. Then after i'm flush with the tape i go slowly without it. then i give a rough shaping then i fine tune with the last4ever tool.
A lathe is the best but most don't have access to one.
Been doing tips by hand since 60's, and then for years had a lathe for others and myself.
I use the $20 Mori/layered/hard tips, I don't mind the time it takes.
40 yrs ago at pro events, allot of the W. Coast Mex players would hunt me down (before the times where lathes were at events) because I would put one on for $20, and the La Pros were about $1 a tip, so the time was worth it.
This was in the day/vegas you could get Prime Rib dinner for $4.
Like you said....razor blade.
But it must be
NEW. Why?
Because as your pushing down on the cut, a dull razor can/will pull/accordion non-layer leather/expand it, and now a bad tip.
If you have a bad tip, you'll know right away with a razor.
On a lathe tho, I was not able to feel the leather as easily when it was bad/dried out, a house cue, no biggie, ball bangers it doesn't matter.
While I'm watching TV I'll vertical the shaft on a hard surface, then trim the sides. VERY SLOW.
Like Zerk said, to not nick the ferrule. In my days they were usually Ivory.
Spending 1/2 hour getting your tip/ferrule right is worth the time.
I've used Super Glue Gel since it came out.