I always return extra change. Most of the time, it's going to come out of the cashier's pocket and I'd feel terrible if I took money from someone like that.
At Lowe's a couple of months ago, I bought a $90 battery charger along with about $500 in building materials. When the total came to about $100 less than my mental math, I pulled the cart over and checked the receipt. They didn't charge me for the charger. I recognized the opportunity to gut-check my 18 YO son to I told him, "check it out, they didn't charge me for the battery charger" he immediately said, "well, let's go back". I'm soooo proud of him. (He's in college to be a police officer right now.)