I've noticed that I have several versions of the MS Visual C++ redistributable installed (I think they most likely installed with some plugins/apps).

For the sake of keeping the PC as clean as possible...

I have 1 x 2005, several (different) versions of 2008, 1 x 2010

I imagine I need to keep the 2005 and 2010 (or are they backwards compatible) ?

But for the 2008, is it ok to uninstall all but one (possibly the latest version) ?

Remove nothing. They have been installed by apps that needed them. Most PC problems are caused by tinkering

Redistributables are needed when an application/dll is linking with the common MSVC runtime library - the basic calls for C/C++-language you could say.

Microsoft made some security fixups since some years back(VS2005 arrival) when using this common runtime.

If all apps were to link with static runtime linked into the executable the redistributables are not needed, since it's not shared among other apps in computer. So one could recommend developers using this technique instead.

So what's already said - yes you need to install them all and it does not harm doing it many times.