Jake brake on or off driving on flats or small hills?

How about 50/50?
Not trying to be a smart @** but I find that I do toggle mine off and on depending on traffic and roads/terrain. I definitely like to use just the foot brake in stop and go traffic, for highway I usually set it to use the engine brake.
I also follow the recommended process for testing brakes and have (not always but often) measured my slack adjusters to ensure that they have the proper travel.

Chris