So, I have a tacx flow smart that I use with a Cinelli semper and my problem is that is almost impossible to perfectly center the tire. I have no spacers on the left side and yet the tire is on the right side of the metal roller. The tire is still fully within the metal cylinder, though.
As above, as long as the wheel is perpendicular to the drum, it is ok. If it is not, it will pull to one side and wear the tyre quicker and may also cause extra drag. It may happen when one of pivots holding the bike sags or incorrect skewer is used.