I agree with much of what Pat has already said about what the Pirates should do with their rotation now that they've clinched a playoff berth, so this will mostly be a "what he said" post, but here's my take.
The Bucs have clinched a playoff berth. They're a game and a half behind the Cardinals. If the Pirates win their division outright, they'll avoid a must-win Wild Card game, which takes place Wednesday. If they tie with the Cardinals, they'll have a one-game playoff Monday, the winner of which would head straight to the division series and the loser of which would have to play a Wild Card game, almost certainly against the Giants.
The Pirates are juggling competing goals. On one hand, they should try to win their remaining games, so that they can win the NL Central outright, force a tiebreaker against the Cardinals, or, if they have to play a Wild Card game, at least preserve their home-field advantage. On the other hand, it's currently very likely they'll end up in the Wild Card game, and if so, Francisco Liriano or Gerrit Cole should start. (Pat quickly dismisses Edinson Volquez, and I agree with him on that -- Liriano and Cole are in a different class.)
The problem, of course, is that Cole started last night and Liriano started the night before. The Pirates' current schedule would have Liriano starting Saturday and Cole starting Sunday, which would rule them both out for the tiebreaker on Monday or the Wild Card game on Wednesday, unless Liriano were to pitch on short rest on Wednesday.
The Pirates don't have to make a decision now. If they lose their next two games and the Cardinals win their next two, that would be a drag, but at least then the Pirates could plan to have Brandon Cumpton start at some point this weekend and have either Liriano or Cole take a day off, or partially off, and do what he needed to do to prepare for Wednesday.
But let's say that doesn't happen. Baseball Prospectus thinks the Pirates have about a 10 percent chance of winning the division. That's not much, but it's meaningful, because winning the division allows the Pirates to avoid a Wild Card game that would cut their chances of winning a World Series in half. By the weekend, if the Bucs' chances of winning the division are, say, 30 percent, I think they'll have little choice but to start Liriano and Cole over the weekend and let the chips fall where they may. The advantage they would enjoy as a result of winning the division is just too big to pass up, and they probably ought to go for it. That might leave Liriano pitching on short rest in the Wild Card game, which worries me a bit given the number of walks he's issued recently, but there's a ton of upside in taking a chance at winning the division.
If the Pirates' odds of winning the division are still around 10 percent or lower by the end of play tomorrow, though, I like Pat's idea of having Liriano piggyback a start with Cumpton on Friday. Liriano could pitch a few innings on short rest, then start the Wild Card game on full rest Wednesday, if needed. Cole could pitch Sunday, then pitch Game 1 of the division series next Friday if the Pirates win the Wild Card game.
If the Pirates did end up tied with the Cardinals, they would probably have to start Jeff Locke, or maybe Volquez on short rest, in a Monday tiebreaker, but I'm not sure I see a way around that, so it's better to not worry about it. It clearly isn't rational to try to set your rotation for a tiebreaker, since you might be able to avoid the tiebreaker by just winning before that. Also, as Pat points out, winning the division would likely force the Pirates into a series against Clayton Kershaw and the Dodgers, and so, as much as the Pirates obviously should try to win as often as possible, there's at least a silver lining if they were to lose the tiebreaker game.