I think it is a good habit to avoid the 'because the board says ...' prompt for decisions. It is a very seductive short cut but it also keeps the other folks out of touch with the trade-offs. In an early startup I was in, I got to see how it affected people (it makes them feel like they are robots or serfs).

The truth is, if you know that the board said to do something, you were also there at the conversation about it, and in that conversation are important bits about tradeoffs and how things were considered. Telling someone "After considering X, Y, and Z, the board and I decided that Q was going to be our best course of action."

I have also seen it used as a shield, as in "the board said we have to do this" which the implication that the person making the request (typically the CEO) didn't agree with the request but was overridden. But the truth is that in a start-up, and even in a larger company, the board is often unwilling to override a CEO's solution to a problem, in favor of one of their own. Good board members respect that the CEO's day to day vision of what is going on carries a lot of weight.