I do not ever give this information out when making a purchase. I find it off-putting to be asked, and I also resent the fact that this slows each and every transaction down. Even if there is a line of 20 people at 1 register, they will still ask every single customer and as a customer in line I resent that. It is not necessary for the transaction, it's for marketing. My transaction shouldn't be made slower by their future marketing. So I don't participate. I just say "I don't give that out". A couple of times I've gotten the "are you sure, we will send you coupons" line but anyone who wants coupons knows how to get them! I get coupons already for any store where I will actually use them. You can sign up for anyone's coupon list yourself on their website, at home, without buying anything or holding up a line.
Yesterday I got asked for my email address.....when I had just paid using a coupon that was sent to me over email. I could not help but point out that they already had my email and that's how I got the coupon I just used. She asked if I had ANOTHER email address I wanted to share. Really, I think one "no" should suffice. Don't hold up the line!