Your bow arm elbow should never be locked straight.

With that said, different people have different degrees of bend in their bow arm.

The most important things to remember are: whatever your form is, it must be able to be replicated time and time again, it must be comfortable to you and it must produce the same results (aka arrow flight) each time.

I've seen some archers have horrific forms, but they can shoot the lights out. That doesn't mean they are wrong!