I used this technique a few times until I realized that spammers and, most importantly, companies that sell their user databases also know about it. So it's actually pretty trivial for them to strip the +something bit before any shady business. Now I don't really care anymore. Most spam are catch automatically anyway. Even when they're not, it's actually a tiny annoyance to me. And what am I going to do if I know for sure that some company sold my data? Sue them? I will most certainly not.
Gmail explicitly ignores '.' in email addresses. I don't think it's standard behaviour across other email systems, though, and even gmail didn't always ignore it.