They should show (and teach) this during sex ed. I'm sure teenage pregnancies and STD's would go down across all highschools!
I first heard of this from an episode of South Park where Mr. Garrison demonstrates it to a bunch of little kids during sex ed.
I thought that it wouldn't be a good idea to do that after you blew the guy for a some time, cause the pre-cum would be in your mouth and thus, all over the outside of the condom. So it would have to be like a right away thing, no bj before or anything! And where is the fun in that?
It's funny how deep South Park and porn can make you think.