I can never keep axles straight for very long on 7-speed freewheels. They just don't hold up to even small bunnyhops, curbhops, etc. That's why I think it's a waste of time to change axles, unless you find someone who'll sell you say a box of 20 for $1 apiece.

Long term, I think I'd look into getting a wheel or wheelset with a freehub and 7-speed cassette, they hold up to my riding style much better. Perhaps they would yours also.