Are you talking about just interior linemen? Because skill positions obviously don't adhere to this rule (WR hands in front or at their side, stand up TE, RB etc.)
I was a blind side tackle, so I'm coming at this from an interior lineman perspective. I'm aware eligible positions don't have to get into a "down" position.
As long as you are "set" and on the line of scrimmage I don't think it matters. We always had our tight ends standing straight up.
That's the difference, though - TEs (as long as they aren't covered) are eligible receivers and can be set in a standing position. I'm not sure if this is the case for linement.