A nice thing about an angled CP is that it makes the controls more accessible to people with different heights, and like howard said, the deeper the CP, a steeper angle will be needed to reach the controls farther back by the kids. Also the higher the CP on the cabinet, the steeper the CP should be. Too much of an angle, however, is hard on the rest of the people's wrists. 5-15o is a good compromise for most people on most arcade cabs, but there are real cabs with flat and the 20o+ CPs.
In my playing, norrow CPs seem to have the biggest leeway in "okay" angles
FWIW:
HotRodse CP angle = ~10o
my BYO CPs (non-cab) angles = ~7.5o, ~8.5o, and ~10o
I also did a test CP flat (no angle), and I hated it.
I am planning my upright cab to have a 15o CP, the same as the atari system2 cab I am trying to copy. Note the atari sys2 cab seems to position the CP a little higher than most others I've seen (~40" front vs the "normal" 36).