Wait, did he mean seams in the subfloor? Because that's the only place that rule really matters. For deck boards or flooring, you actually want the end joints right over a joist so it's supported. But subfloor seams should be staggered between joists so you don't create a weak spot across the whole floor. Maybe he just mixed up the advice.
Wait, so the old guy at the lumberyard told him to keep seams off the joist line? That sounds like a great way to end up with a trampoline in the middle of his kitchen. I guess some advice is best left at the lumberyard.