It depends. There are some advantages to having other peoples sites on your account:
easier to access and manage
you can charge people a small fee, and help balance out your own hosting costs
If something goes wrong..they will be calling you and expect you to deal with it
if you have a falling out with someone..you still have their website to deal with..
Some of my regular clients who I know really well I host on my own account. Other people I just set them up with their own account, and use the DreamHost promo to give them a discount on their new account.