Depends on the contest. Most PG-13 contests generally don't allow them, but could make them pass depending on the amount and overall lyrics. Other contests are fine with it. And then there are the R-18 contests which have showing hentai as their point, so I doubt a f-bombs would be a concern in that case.

To wrap it up, the best thing you can do is actually ask in the contest(s) thread(s) itself(themselves) and see the response.