Most cards with borders on 4 sides (that contain all the card information within those borders) have the same distance from the border to the edge of the card. Geometrically speaking, T206 cards are centered if the distance between the black border and the edge of the card on the left, right, and top side is the same, therefore the bottom of the card is centered as well by definition (assuming non-trimmed cards). The proof is left to the collector. (Though some well trimmed cards could be centered as well)
If cards are trimmed inside the borders, these are known in the medical community as "doctored without borders" cards.
