Probably the best place to start is adjusting all gib strips then tramming the vertical head in the normal manner (dial indicator swung in spindle, bearing on the table at four points). Then try machining some blocks of metal and measure how they come up.

Not a lot you can do if they come up out of spec without a major rescrape etc. But if it is little used, that should not be a problem.

If your arbor socket is running out of true, it might throw the arbor out enough so that cutting is all on one side of the cutter when cutting grooves or steps etc. Probably no big deal when facing. How much out of true is it running?

No, any potential, which might be found when you measure up, will result in the oversize cut (if important) and as stated, the cutter may cut on one side more than the other, but it cannot present tapering problems, like on a lathe. You are not cutting cylindrically, as on a lathe. Setting up the workpiece will be the important factor in achieving a parallel and square finished article.

Important cuts in this context might be boring holes or making slots.