Letting a customer bring in their own is always best, but it pays to keep a good selection of different types of music on hand. I have a couple of disks I've created for demos, but I keep about 30 CDs of various stuff for people to choose from. This is the track list of a disk I burned for demo purposes a couple of years ago that is the primary one I use:

Allman Brothers - Whipping Post
Guns N' Roses - November Rain
Shaniah Twain - Don't Be Stupid
Jewel - Foolish Games
Top Gun Anthem from the Top Gun Soundtrack CD
Joe Satriani - Summer Song
Led Zeppelin - Stairway to Heaven
Stevie Ray Vaughn - The Sky is Crying
Leo Kottke - Jesu, Joy of Man's Desiring (Bach)
Cherry Poppin' Daddies - Zuit Suit Riot
Pink Floyd - Time
Coolio - Gangsta's Paradise
Celine Dion - My Heart Will Go On (Okay, it sucks but it helps to sway the wives now and then.)
Metallica - Unforgiven