The best "safe" solvent I've found for removing glue/tape/sticker residue is De-Solv-it. You can get it at any hardware store (or Meijer's in my neck of the woods). It claims to be 100% Organic and Biodegradable. I also use it as my "go-to" parts cleaner. You do need to follow-up and wash off the parts with some detergent or 409 or such. No fumes, not hard on your skin, not toxic, it's really handy stuff if you tend to do your cleaning indoors (as I do). It came in real handy just yesterday in removing some stickers from some presents. It won't harm plastic.