You never noticed any gaps after a dozen rushed cuts? My old rental had one bad jamb cut and you could see daylight under the baseboard from across the room (it was that obvious). I had to shim it up with cardboard until I could fix it right. That single experience made me a total believer in taking the extra five minutes with a level now.
I've rushed a dozen jamb cuts in my old apartment and never noticed any major issues. Maybe it depends on the type of flooring or how out of square the frame is.
Honestly, I used to believe the same thing until I worked on a place with uneven hardwood floors. Cutting the jambs without accounting for the slope left visible gaps once the trim was on. The baseboard didn't sit flush at all, and it looked terrible. Now I always check the frame with a level first, and it saves a lot of hassle later.