Always protect the "core" of your business. It really doesn't matter who the developers are. Your first responsibility is to ensure that whoever is writing the source code has a strong and long term relationship with you and your business.

Take the selection process very seriously and check referees, talk to previous clients or employers and complete due diligence very thoroughly. Good coders, for example, should make plenty of comments within the script so that anyone who comes after can pick it up and run with it quickly. I have seen many businesses suffer because of a fall out with the coder who didn't do this. A new coder comes in and wants to rewrite!