I've always struggled for perfectly level horizons
That's what post-processing is for! 
See, now that's another area where I am a total and complete luddite. If it's not good full-frame on the negative or slide, then it's not good!

The extent of my post-processing is tweaking the levels a little bit, and it's only been within the last couple of months that I've even started messing with that. I guess I need to get with the program, so to speak...

John