Guys this is all great advice. (I can't believe someone hung a door
with only the trim nails! Yikes!) Yes, when I did my one and only door
last year (my dog ate it!) I thought to hang it on the hinges first,
to make sure I got the spacing right, but then I marked it with a
pencil, routed most of it out, and finished with a chisel. But I like
the idea of using the Stanley knife! I think after hearing all this
I'll go with just doors, not pre-hung. For one thing, I'll save a
couple of hundred bucks over getting pre-hung doors (and 12 pieces of
casing). I can use the $$$. And depending on the weight of the doors
I'll consider putting in a 3rd hinge - another good piece of advice
that I wouldn't have thought of.

I don't hang out on this group much but whenever I do, I'm
consistently impressed with the quality and quantity of advice.