I give my mower a check-over every year, it's a Flymo. I take the blade off and put in a vice and use a file to get rid of any damage caused by hitting the odd stone, taking care to retain the same angle, then finish off with a belt sander with a fine belt.
If I centralise it in the vice I can check to see if both sides are straight. If not I wack the one that isn't with a hammer until it is.
Depends on the mower. On mine with the long bolt that secures it to the machine, there's a bit of play between it and the hole in the blade.
So I don't quite fully tighten the bolt until I've turned it on for a second and let the centrifugal force centre it. Then tighten it fully.
Works a treat.