That's a good point, and ideally Brewers win every tiebreaker, but let's talk through this.
First, I think we can discount the Phillies - we've got just about as big a lead on them as the Cubs - magic #15. They could still get the tiebreaker on us, but it won't matter. They are, however, in a position to possibly win the division, but that would also mean ATL losing a lot and therefore lessening the possibility the Braves could overtake us. The Phils have 7 against ATL in the next 10 (and Sale vs Sanchez tonight) games, and have series against us and the Cubs coming, too. I would not bet on them.
Now let's look at ATL. Currently, Braves are 28-14 in their division (10 division games of their 22 left) and Brewers are 24-13 (15 NLC games in final 21). In order for the tiebreaker to matter, ATL has to make up 3.5 games. I can't do the actual math, but chances are that if ATL and MKE are tied at the end of the year, that means that some combo of ATL winning their division games and MKE losing theirs has happened, and therefore, it's likely ATL gets the tiebreaker.
What I take from this is not worry about the tiebreaker with ATL and just get the magic number to zero without it. Currently stands at 19.
Maybe that was only interesting to me...