This doesn't just apply to developing, or other stuff you're particularly good at, either. In general, people suck at stuff.
My experience tends to be that if, as a hacker, you think you've got an okay grasp of a subject, you're probably in the 90th percentile somewhere. If you think you're pretty good at it, probably 99th.
The "keep raising your rates" advice is good for life in general. Never, never sell yourself short on anything.