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 email@example.com. So if I was a wary customer signing up for Blogger, I would enter mine as firstname.lastname@example.org.
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.