Nobody is going to provide you with all straight 2x6’s or 5/4 boards out of a bundle. (Alfa lumber in Birmingham provides the best no.1 pine you can get) Especially 20’ long. That’s what a small nail bar to pull boards straight before you nail or screw them, or “pull nails” are for. (That’s a nail angled in the side of the board to pull it straight) Your decking depends upon the joist center width. If you’re building a regular deck with no hot tub or anything else super heavy on top, then 5/4 boards with joists on 16” centers will be fine. With 2x6”s you can move your joist out to 24” centers. A 2x8 joist should spam no further than 12’ without a girder. Just FYI.
You can thank these rapid growing pine trees for knotty, crookedass boards.