Well, in an automatic it's not really going to matter until you get up
through the first 3 gears. At that point, you will ACCELERATE faster by
staying in 3rd as opposed to jumping into overdrive....but your speed will
be limited by that gear. The only way to get a higher speed once your maxing
out rpm's is to pop it up to overdrive.
On 3rd gens, and I am only assuming this still holds true for 4th, if
you drive in D you will not go into overdrive if you are giving it more than
2/3 throttle. Also, if you are already in OD and floor it, it will bump you
back down into 3rd to give you that immediate high rpm boost.
Whether you should bother to keep it in 3 or not totally depends on the
race and your driving style. You might need that OD if you are going to be
at higher top end speeds in a long race. If this is just a short
straight-away, though, probably no harm either way, as long as you adjust
the throttle appropriately. Just be glad you have an automatic if that's the
scenario.