Can you assume that a board is straight, just because you’ve jointed its full length? Nope. It can still be as bowed as ever.
You really should check it before moving on to the next piece. Rather than pull out a long straightedge, I just place the board against the jointer’s fence and look for gaps.
If I spot a gap at either end, I know that I shouldn’t press down on the leading edge on the next pass, and listen when the jointer’s knives start cutting. The next full pass should do it—but I check again, anyway!
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