
While I don't speak for Apple, just like the previous responder, you'll find that this is generally not a problem if you…
1) Don't complain about is cosmetic damage caused by the application or removal of the decals.
2) Don't apply them in sesitive areas such as over or near fan grilles etc.
3) They aren't heat retaining although if you stick to the top cover, which is most likely, then you've got little chance of this being a problem anyway.
Just be aware that if you send your MBP in for repair and if any part needs replacing that has a decal on it you aren't going to get that decal back. If you are concerned with your warranty, which is a good thing, then there's no harm in giving AppleCare a call and ask them directly on their policy(ies).