I trim my hair by doing a rollerset. Because my hair is so curly it is the easiest way to really see what my ends look like. The first time I did it I used a small tooth comb and where the comb stopped, I snipped everything beyond that. Now I I can see what the ends look like and snip from there.
My 4 b hair is pretty straight from a rollerset. I smooth my hair and use paper to smooth the ends more.
I wouldn't do this to get a blunt cut or anything like that. I think the hair should be completely straight for that. But if you usually wear your hair in curls or in layers I think it is fine to use a rollerset to do it that way.
If you want a blunt cut I would think the best way is to have it flat ironed or at least blown out very straight and then trimmed straight across. Otherwise I think it would be unevent.