Hi, 90’s MTB guy here:

Generally, on a rigid, it wasn’t a whole lot different than a road bike, but often we’d run the front lever on the right, so you could put it vertically behind the fork blade. (Especially East-Coast woods bikes)

Rear QR would usually go inside the triangle, just above the chainstay, so the lever tip would be “in the shadow” of the frame tubes. Oversized chain stays and chunky dropouts sometimes prevents this, but straight back was usually the last resort; you don’t want anything unnecessarily sticking out, in any direction.