I don't worry much about fore and aft centering at the partners. But if you have an adjustable step, you ought to be able to reduce or eliminate any asymetry there as desired.
Failure to get athwartships centering there could well indicate a subtle bend to the mast. I've always taken that out by wedging. You can't do that with poured stuff, only real wedges made for the purpose.
Before deciding, I'd check the scantlings and the rig design - staying and all. On many boats no weight advantage is taken by having keel stepped and the mast walls are just as hefty as deck stepped masts need to be. This may be to achieve extra strength or because of what's out there on the extrusion market or builder indifference or rerigging along the way.
Do you have a mast and rig designed to take a bend, or is your main (I'll ignore the mizzen for now) really designed as a straight column? Even if the latter, I'd be inclined to force athwartships symetry at the partners if I was dead certain that the hole and step are both themselves perfectly centered. It'd hard to see a subtle curve but there have been columnar masts where I could sight from the step through the partners and see a curve. If it came out with a bit of creative wedging, that was nice. In other instances, the tune was right even though the mast leaned against the foreward side of the partners hole. If no adjustment in the step, I'd wedge it thus.
Part of my prejudice about poured wedges is the nightmare they cause in stepping and unstepping. Most boats with aluminum masts (parallel sided extrusions in that area) also have parallel sided partners. So getting things in an out is a royal pain. One boat that was a constant horror I finally solved by shaping a very subtle taper into the partners such that the poured wedge only got really firm when gently tap-seated after the mast was in the step. Comes out more nicely as well.
G'luck
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