It depends. How much you risk should be based on what you can afford to lose. If losing that crew member means not being able to meet other voyages for the day, or the resource loss is going to slow you down more than just not going on the trip, then don't.
Well, I had a really bad port day today. Yesterday I didn't fail one mission, today I failed four (one in the 80% range and another well over 90%). Plus one bannerman dead after a 67% trip. I just sent out my last mission at 53% - couldn't reach the 7k requirement even with good crew and all the ship updates available. Anxiously awaiting the mayhem that will come of it. You win some, you lose some, I guess.
I wonder if it's better to skip the mission when you can't reach +70% or to take the risk rather than gain nothing at all. What tactics are you guys following? Doing all the missions, or do you guys leave some open at the end of the day if they're unattainable?
At some point, it becomes probable that you can progress faster by not completing all your missions.