Not sure what you mean by flat grain. One type of board is flat-sawn, rather than quarter-sawn, or rift-sawn. Vertical grain is preferred over boards with grain "run-out"...where the grain runs to the edge of the board. This can result in splitting at the location where the grain runs out. Knot free, vertical grain boards are used for dimensional lumber in boat building, for the most structural components like gunnels and frames. Floor boards could have some defects.
Yes, grain run-out will matter even if the board is not bent. It leads to a structural member with less strength. If a frame has run-out and the boat gets hit from the side, it will split where the grain runs out. Bending will make matters worse.
Eric is right on. In my experience with inwales/outwales grain run out less than 24" is risky- depends on species and if you try to steam bend. My floor boards or rough sawn eastern white cedar have knot holes- no problem.