I once took over a marketing position from someone who had created the client’s social media accounts using his personal email address. This was a serious client, 150 employees, and every social media account was tied to the personal email address of some random guy who the company wanted to fire any second, but his “ownership” (from the technical implementation point of view) of all the client accounts made it excruciatingly difficult.
What the original marketer did was unfair to the company. It put him into a blackmail-like situation in which the company had to treat him too-generously because he held the keys.
So, the lesson here is, on day one, don’t even give yourself the keys! Do everything via the client’s email address.
And if he doesn’t give you one, then create a new email address and do everything from the ground up with that new email address.
That way, if you ever step down, you can just give him the user/pass to that email, and voila, no problems.
There’s an even better way to do this, tell him you’re doing it. Your client or boss probably never thought about this case, but by mentioning to him that you’re doing this, he’ll appreciate it, and he’ll see you have his best interest in mind.
There is one exception: Facebook. Facebook requires that each identity map to your real identity once and only once, so you can’t create a new Facebook account. The most common way to deal with this is to share Facebook passwords. But that commonly leads to Facebook locking people out (hmph, how come this one person is logging in from two different cities simultaneously?) so I’d suggest instead going through the arduous process of giving your Facebook user access to the other Facebook account. Frustrating, but worth it to do. And for other systems that have a “one user must map to a real person with only one account allowed per real person” I’d suggest going through the same gymnastics as well. A little pain now saves a lot of frustration later.