Kirk Wight wrote a good post about this a few months ago: Jetpack and WordPress.com accounts

tl;dr: You'll want your client to create their own WordPress.com account if they don't already have one. If you connect Jetpack to your own WordPress.com account, they will still have to link to their account to use features such as Likes, Notifications, or Post by Email.

It will also make things easier if they ever need some help with their Jetpack site and don't want to go through you.

It's also worth noting that a lot of people may already have an account, if they signed up for Gravatar for example.