i did a white oak frame as a porch, my first solo timber frame, back in 2000?.
the posts sit on sills, i drilled 1" drain holes in the bottom of 1.5" mortises (should have made them through and been done!). sealed everything with anchorseal - end grain inside of mortises, tenon ends, shoulder end grain, etc. midwest in the summer, and saw a difference in checking from the timbers i prepared to the extras i ordered that were cut to rough length but not sealed.
in general, now, if i can get away with the design on an exterior stucture i do everything i can to avoid post on top of sill and into a mortise (unless it can be a through mortise). at one point when at NEW we were even designing brace mortises and housings to be angled down, to let water that may have run into a post from the top of the brace, to drain.
