When registering for an account on any online service it's always best to check the site's privacy policy to see what a site will do, or will not do, with the details you provide.
Personally I can't be arsed with all that
fannying around so I just fire in my details and hope my spam filter is up for the challenge, and to be honest,
Gmail's spam filter is very good. A way in which you can find sort this out is to sign up with your email address as
username+sitename@gmail.com. So if I was a wary customer signing up for Blogger, I would enter mine as
graham.macleod+blogger@gmail.com.
You can even have these filtered accordingly, by going into Gmail and select
Settings, then
Filters and
Create a new filter. Add in the address which you signed up with into the
To field, and on the next step select Apply the label and then create a new label, I.e. Blogger.
If any spam comes in, you can view which email address it was sent to and therefor tell which site's
admin's balls you can go and break. Make sure you don't select Delete it at step 2 of the filter making process as you will still want to receive authentic emails from that site.
This doesn't always work and sites with more advanced registration processes may not accept it. It can't hurt though, unless you have had to enter a mountain of personal info and because an error was detected, you have to start again. But in that case, the website is shit and you should think twice about registering in the first place.