I'm cheap. I rinse them. I'd probably think twice about re-using it on wood if I had previously used them on metal, though. For what it's worth, I cut them into long strips (width of the page) about 1" wide, then cut off the used end when it no longer seems effective.
Mine are used on CA so I get a white powder build up. I put some used ones in a laundry bag and my wife is washing them. We'll see how that turns out. I think I'm fiscally conservative (my kids say cheap) and don't like to throw away things. Waste not, want not was preached to me while growing up by my depression era parents.
Meh, I chuck it once it's all gummed up with the white-ish powder from polishing resin. Time is too precious and valuable to me to spend on washing sandpaper. It's a disposable commodity in my book. Just my .02¢