Need a "common sense" breakdown showing you what characters should be allowed in the username part (local part) of an email address? This handy guide from Jochen Topf covers exactly that.
It doesn't EXACTLY align with RFCs, but when you look at it from a common sense perspective, I agree with his categorization of each character. This would be a good thing to reference if you were building your own email capture form. (I'd probably also reject the "maybes" for an email capture form, but not reject them in an MTA configuration. Some of the "maybes" show up in bounce addresses somewhat regularly, but are almost never found in legitimate end user email addresses.)