Hello,

We were a little bit hesitating first couple of times with our customers in terms of asking for App Store credentials and wrote instructions for them since it's really private. But it turned out that most of our customers suggested to share credentials with us not to spend time on publication process that I would not call obvious. Since then we started to ask and work with customers' credentials completely maintaining their accounts.

Like it was mentioned in the previous answer -- outsourcing is a different model of business assuming a bunch of activities aimed at serving client's development process. Having own app that we develop and sell by ourselves I would say that these activities are completely different and requires different skillset that just iOS development. So I'd say that yes it looks to me like a common practice.