Statistics PhD - Sending the math GRE score mGRE is only recommended when applying for a Ph.D in statistics. 
But obviously sometimes it's better not to send a score than to report a bad score.
Stanford, which is the only place that actually requires mGRE, post that their average is the 82% quantile. But this also includes graduate students in probability, with much higher level in mathematics. 

How would you cut the threshold between send/not send/indifferent?
It really depends. Obviously if the score is required you need to send it. If you are applying to top programs, a 68% at anything is not going to help your case so I would not send it if it wasn't required. At lower ranked schools, a 68% might be okay, but again it is probably not going to impress anyone. I would probably draw the line at the 90th percentile for providing something that is not required.