It depends on what sort of clients you have. Are they technical enough that they would want sftp (or ftp) access, or will they be happy only having the WordPress menu to work in.
If they are the type that want to get in and edit code and such, then you should have a different server account for each website, as LakeRat mentioned.
However, if your clients are the type that get glossy-eyed when you discuss code; then you can easily have a single WordPress install set up and enable Multi-sites. That way each client can have their own website, and you only have one WordPress install to deal with.
I currently run around 8 WordPress websites for some small businesses in my area using a single install of WordPress. I charge a low yearly fee for each website to cover the cost of the entire account with DreamHost. Works great, been running it for a few years like that with hardly any problems at all.
Again, these are all just local small businesses with low web-traffic. If someone had an e-commerce site or tons of traffic I would probably create a seperate install of WordPress or whatever they were using to run their site.