First off, never just grab a girl and try to dance with her. Establish rapport with the girl for at least 5 minutes first, in an area away from the dance floor. Just grabbing some random girl has a 99.9% failure rate, unless you're a celebrity or a pro athlete.
Second at the night club I work at, the bumping and grinding is usually reserved for the kids in the club with a Fake ID, or the really drunk easy girls, or couples. Most dancing done by regular adults establishes a distance at which you feel close enough to be personal, but not too close to the point it looks like you're trying to recreate a porno.
Third, a boner should be the least of your concerns. For me personally, I feel it is the girls task to get me hard. She should prove it to me that she has what it takes in the sack. Usually if I don't get one, it means the girl is really bad in bed. If you have no rhythm on the dance floor, what do you think the rhythm is like between the sheets.