Quoted from clodpole:
The steeper you set your playfield, the faster the ball will move downward (toward the flippers) and slower it will move upward. If your playfield has slingshot kickers that face each other, like Flight 2000, steep playfields goof up how those kickers interact.
It's my preference to set the playfield angle lower, to allow kickers and pop bumpers to work the ball back-and-forth faster. Everyone (honest!) who plays my 70s-vintage games comments on how fast they play, and they're all set so the bottom of the cabinet is nearly level.
Experiment with lower angles and see what you think.
It all depends on the pin. I have my Big Game set up at 7%. It plays great. Not so much for Seawitch. I found the Witch orbit shots much more shot friendly when the pin is between 5.5% and 6%. Catacomb works best around 6.5%.