You could always create a second, unused Twitter account for signups to services that don't need Twitter tweet access otherwise.
I have my main account, and then a second one to sign up for these types of websites, services that want to auth with Twitter that might send tweets I don't want (and then I sign up again with my main account if it checks out just fine), contests, and similar. I'm not against Twitter login or contest use, as I can see that being helpful to lots of people as basically a much more accessible OpenID-like single signon experience. I just don't want to spam my followers nor do I want to associate my main Twitter account with that :)
