Optimum tire pressure setting is a very interesting project, that can really give some surprisingly good improvements in handling on some vehicles. It can also be dangerous if you do it wrong, to the low pressure side in particular.
From what I have found (not official) the door post stickers are based on what load those tires could see at their maximum load, which would be axle load for single wheels. It may, or may not, have anything to do with reality in a final vehicle like a B. For example, a Roadtrek 190 on a Chevy has a rear axle rating that is right at the tire rating maximum, so the sticker gives a tire pressure of 80psi, which is the max tire pressure (where it will carry the highest load). The front axle rating is only about 70% of the tire rating, so they put 50psi on the sticker, based on a tire manufacturer load chart I think.
If you are under the axle ratings, you could theoretically lower the tire pressures by some, based on the tire load capacity charts. Many folks, me included, have found that going as low as the charts say can make the vans handle poorly. It is more common for folks to go above the sticker recommendations, if the tire isn't already at max, to try to improve handling. All the Chevy, Ford, and Dodges seem to react well to higher front tire pressures.
There was a pretty good discussion on tire pressure tweaking on the board a while ago.
The front drive Promaster will be slightly different in reactions, but all the same principals apply.
From what I have seen in front driver cars I would generalize:
Front drivers tend to understeer more in most cases, and it will get worse when accelerating or decelerating without using the brakes.
Most front drive, strut suspension, setups have negative scrub radius, so when a tire hits a puddle or big bump it will try to turn in toward the center of the vehicle instead of out like rear drivers normally do. This usually makes them much less likely to move around as much when you hit things like puddles.
Torque steer can be noticeable or not depending on the vehicle.
The attached thread explains it pretty well, but it will be interesting to see what folks come up with on Promasters to get the best handling.