In garter st it probably doesn’t matter that much if you haven’t done any shaping yet, but I would choose a side as front and put in safety pin that side so you can keep track of which side is which.
(If you used Long Tail cast on, the beginning tail for first row would be on the right edge)
When working garter st pieces, just make sure all of them have the CO edge facing the same dir (RS or WS) when you assemble. Depending on the type of CO, some are more obvious than others and you wouldn’t want the bottom edge of one sleeve diff from the other, or the front of the sweater diff from the back. That’s why marking them while working helps to maintain the distinction.