That is a question only you can answer.
How is your math skills?
If you know all your trig functions and what not by heart and don't even need a review of them it may not be worth it.
other than what sin/cos/tan are other very basic trig stuff, I pretty much have to look things up. I found math to be helpful. I got a 17. Believe me I'm happy with it. My brother on the other hand felt I wasted money. But he's a math wiz. when he did the DAT back when it was paper and pencil. he rocked 30 on math. but I beat him in gchem.
The only bad thing about finding out you need it is you have to spend 50 more bucks when you could've bought both math and DAT at the same time.
Personally, its 50 bucks extra. I'm sure you can blow 50 bucks quite easily in a few days I know I can, probably in a few minutes. Why not put it towards your studies. Unless your really cash strapped.