Better to register your own domain name, and give every website a different address. And if you don't need to have a relationship, use throwaway account like mailinator..
"If you sign up for some website that requires an email address (especially if they might spam), using a + alias lets you create a filter for the stuff they send"
It's a great idea...but I'm encountering more and more sites that reject your email address outright if a + is in there anywhere.