I've often debated with myself about doing the same with my Pa900, but usually end up just leaving everything there ... every once in a while I will go through the factory song names/styles and try to figure out what song the style is intended for - although many of them are self-explanatory - ... I will also try out the style any number of ways to see if there might be a song I could use it for ...
BUT, if you really think you will never find a use for a particular song suggestion, then just delete it and 'clean up' the list a bit ...