Should I repair floor joists that have cracks or holes?
Nail holes in a joist wouldn't concern me unless it affected a significant proportion of the joist. You can safely drill a hole 1/3 of the width of the joist in the middle for running wiring.

Cracking or sagging that is allowing the home to go out of level/plumb is a concern. It's best to get a structural engineer to evaluate this problem. The standard solution is to sister a second joist next to the failing one, and possibly add some support columns to a footing in the floor. You would first remove any blocking that's interfering with the new joist, jack up any low points on the old joist until things are level, install the new joist so each end is supported, glue and bolt the new joist to the original joist, install new blocking, and remove the jacks.