Join Date: Oct 2007
Location: corvallis, Oregon
A couple of things. First, for rails and stiles I always begin with 15/16" stock and flatten one face on the jointer. Then I plane them to thickness, trying to take about the same amount off of both faces. More often than not, I will end up with flat, straight rails and stiles. If I detect any slight warpage, I try to put the crowns to the outside face of the doors. On the center stiles, use the 2 stiles that are closest to each other in the amount of curvature. If the crown is outward, the doors can close against the face frame at top and bottom and if you're using Euro hinges, you can usually get them pretty even.
As it's been said, vertical grain is best and most stable, but you won't get that with poplar. Here in the PNW, I can use alder. Soft maple is probably a good choice in your area, as has been recommended. But I have certainly used poplar for paint grade projects in the past and will again, no doubt. It's just those tall doors....