Over the years, the availability of warp-free sticks in the big box stores has been a problem. Because buyers can pick through the lumber. what is loose on the top and front of the stacks has been rejected by previous buyers. I will pass on a purchase if the available stock is junk, but will load up for future projects if I can find a few good pieces. The weeks that pass before use allows the lumber to further air dry and I can reject any piece that warps while drying.
If you grab a batch of 2x4s at the store and start cutting and assembling, you are asking for trouble. Even 1x4s will change shape ... the clear New Zealand pine (Monterey pine) found at H.D. looks great on the rack but can change shape after cutting. Let the lumber rest before trimming it and using it in any applications needing critical straightness. The bigger the stock, the longer the air drying required.
WRT poplar, I have learned to cut oversize and let it sit for a couple of days as it is notorious for changing shape.
