squaring

Hi.

Two of my CNC projects, a gantry mill and a small CNC retrofit lathe,
approach their mechanical completion.
By that time, I need to check if all axes are perpendicular to each other.
I thought about clamping a bar or strip on the mill's table, parallel to one
axis, then sliding a micrometer along a machinist's square to get the other
two right before definitely securing them into place.
Is this a valid practice, or are there better ways to do this?

Also, what about coupling a very small amount of the electronic Z position
value into the Y, and some of the Y into the X, to compensate out the
remaining error through the feedback loop..
assuming the linear slides, which are separate units, are totally straight,
or else the machine might need error compensation tables for each position
in space.

(Fantasy running loose...)
I expect the welded gantry to bend and tweek a bit over temperature and
possibly time.
It would be nice if there was some self-calibrating feature where the
milling head would sweep along some references (thin wires, laser beams?) at
start-up.
Anyone ever been there, done that?

Any constructive suggestions welcomed.

Thanks,
Mike.