Any hints?
A = A very large share of the world's grain provinces somehow have an infantry unit on each of them.
B = You are planning to expand at least ~30 provinces to level 6 buildings.
C = There are only ~5 grain provinces in the world, and you own all of them.
If B you should build trade.
Else If A you should build production.
Else there really isn't any particular reason you should build anything there.
If C is true, building regimental camps becomes viable.
If you're planning on getting certain buildings solely for the demand boost, put them in the poorest provinces you have as a rule of thumb. Demand is demand no matter what province you're looking at. The only exceptions are things like try to avoid building regimental camps in fish provinces or you'll only get half of their demand modifiers on grain, but that's a minor effect overall.