If you're going to an Apple retail store, remember to make a free Genius Bar appointment online before you visit the store, otherwise if they're busy, they may turn you away.
You may also need the serial number of the computer. There's nothing on the mouse that would tie it to your computer, so if they need proof that it's still under warranty, you may need your iMac's info.
Personally, I've never had any problems with the few I've owned. Some people's environments seem to make it easier for them to gunk the wheel up than others, I guess.