Tram is simply establishment of normality (perpendicularity on 2 axes) of the spindle to the table. If your dovetail column isn't vertical, you will (in theory) maintain tram as you move up/down as long as the angles don't change as you go. Your tool tip center point will move in the x, y plane with the motion of the head along a tilted column, but the relationship of the axes should stay the same. If you have a round column mill that does not have perfect alignment with the spindle axis, you lose the coaxial relationship (or at least the instantaneous axial relationship, aka zero set point) as you revolve the tool head around the column. Round column mills need to be trammed (heh, or not) every time you sneeze. If you run a BP style mill, tramming needs to be set every time you tilt or nod the head (by design).
By squaring the column to the table, and squaring the head to the column, you are making life easier on yourself by setting the column and spindle in a true coaxial relationship (this may prove futile). But it is not necessary to do in order to establish "tram", since the typical tramming procedure involves only the relationship between the spindle and table, ignoring the other relationships in the assembly. So while nice, it is not necessary to have a perfectly adjusted relationships between table, column, head, and spindle as long as the spindle and table are in tram.
Some might argue that "tram" is a transverse property, i.e. requires the total system to be in tram throughout its range of motion, but the common "tram setting" process with a t-bar and indicators simply proves the perpendicular relationship of spindle to table at the position of interest.
