Evert 4-player control panel from an actual arcade game that I've seen (tmnt, simpsons, gauntlet, d&d, etc. etc.) mount all 4 joysticks with the same orientation. I've read people mention that it's ok to mount them at an angle, and some like it better, but I'm going to go with the decisions made by the makers of Gauntlet, personally.
Also, mounting your joysticks at an angle will mean that dual joystick games such as Karate Champ Vs, Total Carnage, (etc.) probably won't play very well at all.