I don't have any stickers on my bike (I put patches on my bag instead). I did take all the stock stickers off my Raliegh, and I didn't have a problem. I used a hair dryer to soften the adhesive, and pull slowly at a very sharp angle. A little adhesive was left, and I just used the sticker I removed to get it off.

I'd keep to vinyl stickers (bumper sticker material) and not paper stickers. If you're going to be leaving the bike in the sun, make sure the stickers will be color safe; red seems to fade badly. If you're looking for some commuter themed stickers, check out 1lesscar.com