You're right on about bullpens, if a team has a terrible pen but a good starter then you're better off only doing 1st 5. lots of reasons why people may not play first 5 though. If you're betting on the better team, you have less time for them to prove they're the better team or for their talent to show. kinda like a basketball or football game where any team can hang with another for a quarter, but over 4 quarters the matchup(s) that you expect should play out with more time...
, some pitchers are decent the first time or two through the order but then get hit after the hitters have seen their stuff. I don't like betting national league games 1st five because so many rallies are killed by weak hitting 8th place hitters and then the pitcher...you may only have 1-2 good opportunities for the meat of the order to hit and then boom the bet is over. My local has -1/2 for 1st fives, which is kinda cool but I don't see that option out there much