If you fly IFR in busy parts of the country where direct routings are rare, knowing what routes ATC is likely to give you can be a big time saver. You can look up the preferred ATC routings between airport pairs online to get a good idea of what you should file. My favorite site for doing so is fltplan.com, for the simple reason that I can request an email containing my flight plan confirmation before the flight. Knowing exactly what routing clearance delivery is going to give me before I ever leave the house is fantastic.
It can also alert you to ATC errors, which, although rare, can happen. For instance, on a recent IFR trip, I filed my flight plan based on the preferred routing I found online, but when I received my clearance it was slightly different. It added a couple of new victor airways, one of which was nowhere near my route of flight. When I queried clearance about the discrepancy, the controller agreed the routing wasn’t right, and so he gave me a new one — and guess what? It didn’t make sense either.
