Personally I think it goes as follows. Wheeler in the 9th, Qualls in the 8th and Nieve in the 7th in the high yield innings, when the stros have a lead (like last night). Lidge pitches in mop up duty when ahead by a lot or behind. Think of it like the role springer plays right now.
Maybe Nieve closes but I think Wrecking ball gets that job, and qualls and nieve are set up man a and set up man b. I could be wrong though, it's happened before.