lifeisabeach - so what would you recommend as a good app for removing unnecessary files which get missed or pile up?
Onyx is good enough for most people's need. I like TinkerTool System. It has a few extra tricks available vs Onyx and the explanations it provides are very helpful. It's not free, but very reasonably priced.
I knew of some other utility that had an interesting feature that would seek out .pref files and let you delete anything that hadn't been accessed in, say, a year or more, or however long you specify. I cannot for the life of me recall what it was, or find it now.
But anywho, don't get too hung up on the idea that unnecessary files "pile up" in OS X and must be rooted out to help performance. This isn't Windows with a registry that grows and grows and bloats up to galactic proportions. Any helper files that may not get deleted when you trash an app typically are very small, take up trivial disk space, and have zero impact on performance. There can be exceptions, in cases of apps that hook deeper into the system, like plug-ins, drivers, and such, but those are exceptions and no utility exists that can hunt things like that down for you. It's best to be more aware of what you install and, if necessary, follow the directions provided by the developer for completely uninstalling their software. General rule of thumb: if the app was dragged/dropped from a dmg and didn't prompt you to install some extra support files when first run, then simply deleting it will be enough. If it came with an installer or an uninstaller, use the uninstaller or check for further directions from the developer to ensure complete removal.