In my practice we've always found the relationship to be crucial. When things get challenging, it's usually some aspect of the relationship that we needs attention to get things back on track.
We spend a lot of time on what we call the While We're Still Friends Conversation. Basically the WWSFC covers how everyone will treat each other, how responsibilities will be divided, and how things will be resolved when they get off track.
Trust - often an artifact of a previous relationship - CAN make the WWSFC go faster and more smoothly. It can also cause partners to make assumptions about how things will work. Very often these assumptions do not stand up to the pressures of executing a successful startup.
I STRONGLY encourage you to have your own version of the WWSFC - whether you decide to work with life-long friends or complete strangers. I'd equally strongly encourage you to get professional facilitation for your WWSFC. I could make a long list of reasons why - or you can just trust that having a (relatively) neutral experienced facilitator in the room is the right thing to do.