Most divisional and Wild Card ties are broken by playing a 163rd game. The exception is when two teams in the same division are tied for the division flag and Wild Card, like the AL East in 2005. In this case, the tie is broken by:
1) Head-to-head record
2) Record within the division
Many sportsbooks (and the BP Playoff Odds) do not adjust their odds for this consideration. Based on these criteria, when both teams make the playoffs, the division title goes to:
AL East: Yankees over Red Sox
AL Central: Indians over Tigers
NL East: Phillies over Mets
NL West: Diamondbacks over Padres, Padres over Dodgers, Dodgers over Diamondbacks
How does this affect your bets?
1) Divisional winners. Even though two teams may have the same record, only the one that wins the tiebreak pays out as the champion.
The most important tiebreaker is Diamondbacks/Padres. The two teams should tie roughly 13% of the time (based on their records entering 9/17) with the loser taking the Wild Card the majority of those times. This adds roughly 4% to Arizona's NL West chances, while subtracting 4% from the Padres. If you find a book with Wild Card odds, remember to adjust those by 4% each as well.
2) Playoff seeding
In all of these races except the AL Central, the winner is likely to be the number 1 or 2 seed in the playoffs, while the Wild Card automatically takes the fourth seed.
Obviously this affects home-field advantage, though that isn't a huge deal in the playoffs. However, it also alters first-round matchups. For example, if the Padres end the season tied with Arizona, they must open the playoffs in New York while the Diamondbacks host the much weaker NL Central champ.
These considerations can have a significant impact on pennant and World Series odds, and not just for the teams involved in the tie. Should the Phillies win the Wild Card, the NL Central winner must face New York in the first round, hurting their own pennant hopes while enhancing those of the Mets. If you're a Cubs fan, you should be rooting pretty hard against Philly the rest of the way.
In the AL East, this isn't as important because the Indians and Angels are similarly talented teams. If the Tigers tie the Indians and sneak into the playoffs, whoever gets to face the Angels instead of the Red Sox will be in good shape. Furthermore, if the Tigers do make the playoffs, their first-round opponent is in a great spot, since Detroit's starting pitching is a mess.