If the hanger is bent directly inward, that is with no twist, and there is not deraillsur damage then an eyeball straigtening is generally enough. I know from experience that's not always true, however. Sometimes the deraiilleur is twisted, and sometimes the dropout bend is not simple.

After a trip to Italy I was plaquedy by inconsistent shifting - some gears fine, others lousy and phantom shifts. I was unable to resolve it until I borrowed a dropout alignment tool. I found the hanger was bent in more than one plane, and once corrected I had no further problems. As a mechanic with 20 years experience,including a lot of both frame and drive train alignment I would advise you that if your attempt to resolve the problem fails take it to a good shop.