I'm using HP manifolds in my car so I'll put the coils on the frame rails. With dyno headers the plug routing works out great with the coils on the valve covers.

I'm not hung up on where the coils should go, I think it just depends on the application. When I take my engine to the dyno I like to have everything hung on the engine so I don't have to find a place on the dyno to mount things. If I have the engine 100% self contained then it is easy to just roll it on and off the dyno without messing around for half a day trying to figure out how to hang parts.