Well, there are several reports that were already fixed but are still stuck by the end pages of the tracker. Sure we managed to clear most of them, but there are probably more of them. If you already tested your own reports then you have already contributed more than most people do, as they usually report things and never again bother checking it
I think retesting these old issues is mostly hefty work, but eventually it's also a necessary work, basic maintenance. It's not something you'll dedicate a lot of time to, just try testing as many as you can or feel like. There are also the Feedback issues which require attention too at most times.
Also, checking the change log page frequently helps knowing whether there was a change applied to a given issue or not (these in general should be the ones that need retesting).