Yep! I keep a tripod on the back seat of my car almost all the time in case I need it. There's a steel ruler hanging up on the peg board right behind the enlarger.
I don't have so much trouble using my 4x5 camera. It does have a grid on the ground glass.
Most of my trouble comes because, 90% of the time, I'll be walking on the beach hiking down a trail or walking somewhere where lugging a tripod is a less-than-optimal idea.
If I'm careful, I can keep the tilt down to less than a degree. Unless I really buckle down and, like you said, use the edges of the viewfinder as guides in almost every handheld picture I take, the world is almost always tilted a little to the left.