If not, you should start! I have been saving my receipts since I started this whole coupon game. You never know when you'll find a rebate or a 'try me free' offer on something you just bought. A large factor in my costs staying so low is due to rebates.
Another reason: you may need to return something...like the socks I bought yesterday for 1.48/3pr...I bought them at 50% off when they were 2.98 for 3 pr...so, hubby returned them today for me, to get back over $16. And...the messenger bag I bought at Staples during the 'free backpack' week...although the website showed it was eligible for the rebate, the Rebate Center did not, so back it went! A $74.99 bag was a little pricey for us...oh well!
I don't return things often, but at times like this, I'm glad I save my receipts! My oop that was higher than normal last month will even out this month because of those returns.