Do not pile the mulch on the trunk. Just leave finger room, you can't see it from more than a few feet away but it will keep the trunk dry and prevent disease and possible insect damage. Think of keeping wet socks on for a long time and getting trenchfoot. EEEWWWHH!
Personally, I almost always cut an edge around the mulched area. As a matter of fact, I keep a three inch trenching shovel on my truck for that purpose only. I do use it once in a while for digging around irrigation heads but it's on the truck for edging beds. I understand what you are saying about roots. Yes, sometimes that is a problem. Just do the best you can and here and there you may not be able to keep the mulch as deep as the rest of the way around. Doing this makes a big difference in keeping a nice, clean edge between the mulch and surrounding turf, adjacent sidewalks, etc. I just think it looks professional and I take the extra few minutes to do it. I also tuck and roll the pinestraw at the edges (if you use straw instead of bark) but then I'm picky about what my work looks like.
If it's worth doing, it's worth doing right. If you wouldn't put your name on it, it ain't done right!