Is it a good idea to get a narrower front tire and a wider rear tire on a road bike?
I'm currently running the Conti Attack/Force combo on my road bike. I have previously run both the GP 4000, and the Schwalbe Ultremo ZX on the same bike. 

The concept behind the Attack/Force is that the slightly wider (24mm) rear tire will allow just a bit more comfort, and the slightly narrower front tire (22mm) will be a touch lighter with more reactive handling. 

I feel a difference in comfort, but I can't say the handling was noticeably different. Of course, the GP 4000 was pretty stable to begin with, so maybe that's a good thing that I don't notice a change.

They do feel a bit more stable in a corner, and I feel like the rubber compound is a bit stickier. They say it's the same Black Chili compound used on the current GP 4000, so maybe that's psychological. 

I can say that the comfort aspect is worthwhile, and there is nothing I have bad to say, so why not try them. A set (here, at least) is the same price as a set of GP 4000's. No loss, at least, and a bit of gain on the comfort side.

@DanielRHicks commented that you might need to worry about braking power or traction. I've had no issues with braking performance, and these are a pro peleton tire. I don't see any issue there. I've also had no issues with flats on the tires, but then, I didn't have with the GP 4000 either.