How do I pick 2x4s that won't cup on me?

Two boards developing a bow after a couple weeks nailed up is pretty normal, especially in the Midwest. Lumber sits around at a certain moisture content wherever it was milled and dried, then it has to adjust to your garage's humidity. If it dried out or picked up moisture after you bought it, it moves. Kiln-dried framing lumber usally settles down within a season, and if the bow is slight it won't matter once the drywall or sheathing is on. If one is really banana-shaped, pull the nails, snap it back straight, and renail it or swap it out.

Yeah that tracks with what I've seen too. I framed a shed wall a few years back and had the same thing, a couple studs walked a little after I buttoned it up, and once the sheathing went on you couldn't tell anymore. On the grain thing, what I do is look for the crown, that big arch on the end grain, and set it facing up on horizontal pieces like plates and joists so any cupping goes up into the floor instead of down. For wall studs standing vertical it matters way less. Also give each board a quick eye down the edge like you're sighting a rifle, the wows show up fast that way.

One thing I'd add for the next wall is to let the new stack sit in the garage loose for a week or two before you nail anything, stacked flat with spacers between boards if you can manage it. The first time I framed something right off the pallet the movement caught me the same way, and since then I buy everything, sticker it, and come back to it once it's had a chance to settle into the space. Also get in the habit of sighting down the length of each board like you're aiming a rifle, because a bow is easy to spot that way but twist will fool you until it's nailed in and pulling your corners out of square.

On the vertical studs the crown direction doesn't really matter the way it does on horizontal pieces. What I do with a wall is sight down every stud before it goes in and put all the crowns facing the same direction, usually toward the wall that'll get sheathing first. That way nothing reads as a hump through the drywall. I did a 26 foot basement wall last fall doing exactly that and the drywallers didn't have to shim a single stud. Also flip your twisted ones crown down if they're that bad, no amount of lumber logic saves a corkscrewed 2x4.

Update: took the advice about letting the stack sit loose in the garage with spacers between boards. Gave it about ten days and a couple of the crooked ones actually relaxed back out, so thanks for that tip. One thing I'm still unsure on though. Someone said crowns shold face toward the sheathing side, but my last wall is open framing with nothing going over it. Does crown direction still matter there, or does it not make a difference if there's no drywall to telegraph anything?