Be fastidious in giving every component a face side and face edge mark,
then
use these marks to determine which side of the component goes against the
tool fence. This way, if, for example, your mortice is fractionally
off-centre, then it will be fractionally off-centre _in the same
direction_
in all your components. In the case of the panelled door I've been
describing, if some of the mortices were fractionally off in one
direction,
and some fractionally off in the opposite sense, it would lead to a
twisted
door.

I was going to make this same suggestion... With a modification: I always
index off the "show side" of an assembly (like the outside of a door). If
the thicknesses of stock are off, the offset will show up on the unseen side
of the assembly.

I use 1/4" mortises for cabinet doors. If my finished stock is 7/8" I will
intentionally *not* center the tennon (1/4,1/4,3/8 spacing) so that it is
harder to reverse/flip a non symmetric part. If the difference is
structurally insignificant, don't center your mortises; this makes "front"
and "back" obvious.


-Steve