I tried Guerilla Mail to register a Subway card on which I have over 1200 points (they changed their policy and you have to register card now)... and Subway blocks it as an "invalid email". Bastards.
That five percent is meaningful; and you don't know which ones they'll be in advance. Using this email service I need no spam folder or filter and I receive only the occasional spam, like I did here (and which would always come from an address that I can identify and eliminate). The service I use keeps an ongoing tally of the number of emails that are sent to all disposable email address (that it "eats" as they call it) after they were terminated. As of today I have a total of 124,829 spam emails that I have never seen from those five percent spammers. And that's all good. Signing up for this disposable email service was one of the best and insightful things, computerwise, that I have ever done. If you try it, you'll like it.