Yeah sure, I doubt anyone would try to sell that as a 2x4.

Anyway the point is that the professional carpenters don’t give a shit about it, so neither should DIYers. Once the 2x4 is put up and covered in drywall, nobody will ever know if it has a mild curve.

The actual thing to watch out for are the edges if they are visible in the end project and also cuts that position the knots poorly. I’ve seen 2x4s where a knot went halfway through the width, which would would only hold half the weight that it’s supposed to.