So the material choice has most to do with will it corrode over time, which is what I was leaning to.
Say the foot of the machine is 4x4" but our shim is only 2x2" or even 1x1" Would this be a concern of yours? I would think, possibly, over time the machine might settle in that area more than other due to a increase in pressure from the reduce foot print. I'm probably way over thinking this but makes me curious what others have experienced.
Shim stock is more often cut to stick-out a tad. Loading quite aside, one of the reasons is so that the next Pilgrim who has to check alignment knows it is THERE, and what sort of material had been used.
The "surplus" also serves as an easily visible indicator as to progress of corrosion, if any, and hopefully before it becomes a major problem, overall.
Hardening fillers are not uncommon, either. The putty-like material between the bed and base of a Monarch 10EE may be an extremely thin skim, and there as much to preclude entry of corrosives as to play any real role in altering fit.
The "Bondo"-like filler on my El Cheapo Burke B-100-4 mill, by contrast, was nearly a 1/4" thick, worst corner, main casting to chip/coolant pan to base.
I considered that sort of pretense a bad joke, so used a visible stack of ignorant galvanized "fender" washers when it went back together in a more forthright admission of its humble origins.
In this case, there isn't really anything "critical" about it, as all the moving parts that matter (eg: the spindle, knee, and drivetrain) are hung off the one main casting on a Burke #4.
The 10EE? At bed to base, with three-point mounting further down - where it meets Mother Earth - a precise and stable fit very much does matter.
Your machine. Your own set of priorities as to what actually matters.
If you have a major make with a decent manual, the guidance should be spelled-out arredy.
If not, advice from those with the same machine or closest ones of similar layout and construction can be better than chatter about "general" shimming.
