If you're in the last month, and all visible outward signs says 'pregnancy', then you wouldn't need to at this point.
If you are unsure she is pregnant and going on projected or possible due dates, with a mare who is not showing any other visible signs of pregnancy, you might have the vet palpate her first, and if the vet is not finding any signs that there is a foal inside, then the ultrasound would simply be confirming whether or not she's pregnant.
If she is in the last month of pregnancy, the vet should be able to tell without ultrasound that she is pregnant.
So, my answer would be 'conditional' on outward signs and vet's opinion.