I have a credit card with AMEX (6% Groceries and 3% gas), Citi Forward card (gives me 5% Amazon, movies, restaurants), Discover (5% random stuff), 2 new Southwest cards I opened up for internship interviews this year (4 free flight vouchers and $820ish in SW flights).
I pay it off several times a month, especially for cards I use the most (AMEX and Citi Forward). The others I have to be automatically paid off.
CC don't make any money off of me, in fact, I make tons of money off of them because of the cashback.
I always redeem for Staples gift cards because when I accrue $75 in Staples gift card, I buy a $25 off $100 voucher to Staples, and then go and buy FAR (Free after Rebate), basically converting that gift card back into cash. I get $100 worth of things for free, and if they are school supplies, donate it to the elementary school I work with. I've donated tons of reems of paper, pencils, etc.
My thinking is, if I'm going to buy something, why not get cashback for it anyway. I'm willing to put in the work to make it worthwhile.