I think that each reroll for crew/voyages takes off one possible trip for the day. Lets say you have 12 trips, and 12 rerolls per day (the KB says you get 12 rerolls/day, idk about trips). You can do all 12 trips in order without rerolling, for a total of 12 trips. If you reroll 6 times, it takes 6 off the voyages left for the day, and you only get 6 voyages. It seems like not rerolling is the best idea.