(Note: The following assumes that you know where your default VST2 directory is. If for some reason it's on this list, do not delete the files in it unless they are duplicates of plug-ins that are installed as VST3's)
Some of my favorite plug-in companies use installers that unfortunately, install versions of their products that I won't use and/or duplicate versions. I only ever wish to install 64-bit plug-ins, and I give precedence to VST3's if they are available.
The installers will
Some plugins (waves, etc) don't let you choose a target path. One option that I have found useful is, choosing one location as "the good one" and then replacing all the other folders with directory links to this "good one".
E.g., if I decide to use c:\Program Files\Common\VST2 as the "good location" I would
a) Check c:\Program Files\Steinberg\Vstplugins, and move anything in there to c:\Program Files\Common\VST2
b) Delete the empty c:\Program Files\Steinberg\Vstplugins