My ex-cofounder and I sat down at a table and hashed everything out at the interface level--what HTTP requests to make, how messages were handled, etc. We didn't second-guess each others' implementations, because as long as the interface was stuck to things were fine. That saved us a lot of bikeshedding.

I would say the line is generally "Are we both right given the current business needs?". If the answer is yes, then you're bikeshedding, and should just flip a coin and move on.