My understanding is + sign is in the RFC, so of course it should be supported. If you want to identify your users by email, you should probably strip the + and everything after before checking for uniqueness. On the other hand, dots matter in the RFC and Gmail is breaking it with their "feature".
edit: I was corrected in other comments that the + labeling is optional part of the standard.
edit: I was corrected in other comments that the + labeling is optional part of the standard.