I often use different thicknesses under F/R axle. Never more than just one extra board, and I carry 1.5" thick boards, so if I need another 3/4" to be level, 1 board under 1 axle does exactly that.
I also carry a couple of 3/4" thick boards, so I could level within 3/8" using 1 of those. Never felt the need, and those boards just go along for the ride. But it doesn't cost any extra fare to carry them.