The only consideration I can see for not going too close to lowest MAP is for idle control with AC, power steering load, alternator load etc. If you are too close to MBT and LBT at idle you have no room to take on load.
I log the torque pids and you can make a very accurate looking bhp and torque run with them. So they should tell you about best spark for MBT for sure. Once you get MBT then vary fueling in open loop to get LBT as well.
But as above, this is not what I would do as you then have zero room to vary spark for engine load. You are limited to throttle action which is slow and cumbersome and likely to cause erratic idle.
That being said you are likely to have the lowest airflow at LBT and MBT. Therefore lowest fuel consumption