This depends on the kind of event you are talking about. In Germany a live act in a bar will usually start around the time as advertised. In a concert hall the advertised time is doors-time and only sometimes the time when the act is supposed to start is advertised at all. This might or might not be the time when they really start.
I suppose, that in different countries this will be very different. In fact these are just cultural norms.
Then think of something very famous like Woodstock: We know exactly at which time which act did actually start. And frankly, that is the information I'd want to have in MusicBrainz.
So my guess is, a guideline that says: take any official information you have and become more precise the more information you get, would be in order. "more precise" would always mean closer to the factual time that the artist started playing.