Control panel heights of some games.  Keep in mind that hte hjoy/button locations are probably between the front and back heights.:
Mortal Kombat:
front: 35.75"
back: 38"
Neo Geo (ver 3):
front: 36.625"
back: 37.75"
Simpsons (and teenage mutant ninja turtles):
front: 35.875"
back: 38"
X-men (6 player panel, flat top):
front: 36.25"
back: 36.25"
My opinion:
The front should be 36-37" and slope at 1" for each 12" of depth.  I might do a slight slope on mine but it will only be a few degrees, almost unnoticeable.
Opinion EDIT 1:
As fate has it, I have tower speakers that are 35" tall.  Not a bad height for a common cabinet that many people use where people of all heights would be playing.  But stacking books let me do some height testing.  I'm 6 foot tall by the way and I am planing on an almost level control panel.  36.5" doesn't quite make it either for me.  37.5" is starting to feel doable.  38.5" is what I'm going to go with I think, but I can see why some like 40" heights.  The 38" height makes sense since manufacturers have to make the cabinets shorter in order to serve people of various height.