As Tom said, tramming (getting the spindle perfectly perpendicular to the table) is completely unrelated to the motion of the head around the round column. You generally want the head more or less centered on the machine, but not always. This is where a round column mill can actually have an advantage: Often the round column is a PITA because you lose the location whenever you loosen it on the column to raise it up or down, but sometimes the ability to swing it can be an advantage, as when you have two setups on the table; they can be farther apart than the table's x travel because you can swing the head to put it over either end.
If you tram it when more or less centered on the table, it should still be in tram at other locations. But you will have to rezero it to your workpiece whenever you loosen the column lock bolts.