Contrary to popular belief, you can scrub your cast iron, too. The seasoning (if done correctly) is bonded at a molecular level to the cast iron, so even a hard go with a steel scrubber wont take it off.

Should you do this regularly with a strong soap? No, but its better to scrub off some burnt on bits than leave it on because we think cast iron seasoning will be ruined is we look at it funny.

But if you're cooking with it right, that shouldn't happen too often.