I almost always use a punch because I find it gives a nice clean cut, keeps tobacco shreds out of your mouth and maintains the integrity of the cap. I think it also helps with a nice even burn, and makes the cigar more nub able if it is worth it.
On occasion the draw is too tight, and I cut again with a guillotine. Also use a guillotine for small ring gauge cigars and torpedo's/pyramids. when i do this i try and keep it to the end of the triple cap, or equivalent.
I have a good number of guillotines with a key chain punch attached to them distributed between house, golf bag, office, and suitcase so as to be boy scout ready for any occasion.
One of the guys I smoke with uses a guillotine and cuts the entire cap off, I am surprised he doesn't unravel as he smokes!