Hi there,
I totally understand your nervousness. Your App Store credentials probably feel like the keys to the kingdom. The question you have to answer is whether or not you trust your offshore development team. If the answer is no, you could use a "remote control" app like TeamViewer (http://www.teamviewer.com/en/index.aspx) to log the developers into iTunes Connect. If the answer is yes, you could add one or more of the team members as Users/Testers in iTunes Connect. I, for one, usually hand over my credentials. After 3 years of iOS development, a couple dozen apps, and probably 10 different programmers, there's been no funny business. Here's one last thing to remember: people selling development services in the form of contract work usually aren't interested in becoming app entrepreneurs. They run small lifestyle businesses, not startups, and they actually stand to lose more than they gain by ripping off a client. Even if they do rip you off, I think you stand to gain a lot more by giving your programmers the benefit of the doubt rather than disrupting their workflow to protect yourself. It all comes back to trust. If you don't trust them, fire them. (Also, be sure they're making daily commits to a code repository on Bitbucket or Github so that you always have the latest version of the code.)

Hope this helps, Austin