Really? Is it FMS specific or peripheral specific? I change the altitude and heading values on every flight.
I have both mapped to my Saitek/Logitech throttle quadrant switches…Switches T3/T4 control altitude up/down, respectively; and switches T5/T6 are for heading value increase/decrease, respectively. The longer you hold the switch in, the increments become 1,000s (alt) and 10s (heading), otherwise the increments are 100s (alt) and 1s (heading). I am usually flying aircraft with the Honeywell KAP unit or the Garmin 1000, 3000, and 5000 units.
Maybe, I am not understanding the issue. 