playBunny: I agree it depends on the position, and what knowledge you gain, however I would err on the side of caution if you aren't sure.
pentejr Yes, the %age changes slightly depending on the match score, but not as much as you might think. You also have to account for gammons either way so a simple percentage is not useful until both players have borne off.
What you need to do is work out the chance of winning for each match score (there are tables) and figure it out from there.
So in a 9 point match, ignoring gammons, if I refuse my chance of winning from 0-1 is about 44%.
If I take the double I will either be at 0-2 or 2-0. The chance of winning from 0-2 is about 37% and, conversely, from 2-0 is about 63%.
So if my chance of winning the first game are currently x, then my chances of winning the match if I take are .37(1-x)+.63x = .37+26x
So to equalize to teh 44% chance if I drop, then .44 = .37+.26x
.26x = .07
x = .07 / .26
x = about 27%