If the department doesn't say it's recommended, I would probably not send it. If they say it's recommended or strongly recommended, I would probably send the score if its 40th percentile or up. If it's over 50, I think it's definitely worth sending: this score means you did better than half of all people who want to go into grad school in math. It's really not that shabby.
I also think you should take into account the strength of the rest of your application. If your grades and references are good, maybe a 40th percentile score would make you look worse, instead of better. On the other hand, if your scores are low, or if you never took many classes tested on the GRE, a 40th percentile score could look like a good improvement.