Seasoning checks ARE a grading criteria in some species in higher grades and are limited to 1/4 to 1/2 thickness. Since boxed heart timbers will usually only check to the heart, the check will not go all the way through the timber (then it would be a "split" in grading terms). It's possible that a check could be a split (which also have specified limits) in reclaimed resawn timber, free of heart, I suppose. If that check happened to be right where there was a reduction on the end of a joist, it could be a problem.
