I used to do this when it was new and not many people were aware of it. Nowadays some sites don't accept emails with '+' at all.

Then I setup an forwarding email server, which forward <anything here>@mydomain.com to my main email. So I would register at sites like sitename@mydomain.com. Sometimes, if required to send emails from registered email then I would create a new email and send.

Now, I have switched to Abine Blur and free version works fine for me. They have a chrome extension and it generates random email whenever you are filling the email field in web forms. (or you can generate random emails at will). Only caveat is, you cannot reply from that email. So, in such cases, I login to the site, change my email to something else and compose emails.

I suggest everyone to using Blur or similar tools. The guys who are running your site will ever know your real email.