Many sites require an email-address as user-name, so I often don't have much choice.
I only can decide how important the site is, and if I want to get mails from it. For real important sites (like my bank) I use my "premium" address, for temporary important sites (eg for shopping, booking hotels etc.) I use my main address, and for others I use an address I only check once or twice a month.
And then there are sites where I use mailinator....
so long, Rata