I think it depends on the nature of the relationships. If your startup, for example, is taking on your old company head on, then you may want take this approach

I think it depends on the nature of the relationships. If your startup, for example, is taking on your old company head on, then you may want take this approach. But, and we do this regularly when hiring from past companies or partners, call their boss and make sure it doesn't damage the relationship. You wouldnt want to harm future business with them.