I used to keep all my receipts for everything, and then went to the other extreme and started recycling them as soon as I took them out of the shopping bag. Lately I only keep the credit card receipts, so that I can match them with my statements, but I so rarely use credit cards now that I'm fanatical about frugality.
Almost a year ago I purchased two high-quality furnace filters and ended up using only one of them. Of course, the receipt is long gone (or buried in a clutter-box in the basement) but since we're moving out of the house at the end of the month I decided to return the one unused filter.
Now I know I paid about $30 for it, but because I didn't have the receipt with me I had to accept the last sale price, and on a credit note at that. So now I have an $18 credit at Canadian Tire. I think I have a couple of months to use it, so when I go through those clutter-boxes this weekend hopefully the original receipt turns up, and I can return to Crappy Tire and get all my money back, and then turn around and spend it again on a Christmas gift.
Oh, the joy of de-cluttering. With me, like with most things, it's one extreme or the other. I throw it out without thinking or I keep it forever. Like those clutter-boxes I mentioned. I can think of at least three that I have to dig out, empty out, and hopefully throw out. I can't take all this stuff with me to the new apartment, where I'll be in a bit of a storage crunch.