I try to collect electronic version of user manuals for everything that I own. This is not limited to electronics and computer equipment, but for dish washer, baby gear, anything that I have. The advantages should be obvious:

Less clutter -- throw out the paper version
Fast lookup -- I keep the paper manuals in a big box, but after a while, it is hard to find what you want.
search -- you usually can search within a PDF for exactly what you need.

When I collect, I download them onto my harddrive and also back them up to a DVD. Why not just bookmark a link to the manufacturers' site? Sometimes they move, sometimes they take away manuals for older stuff.