Well, I think I found the solution. If I move the app window to the external monitor, then go to the Dock, right click on the app's icon and under Assign Desktop, select None instead of This Desktop, the app will remember to open on the external monitor.
I've tested this for both iTunes and Mactracker, so far so good and the setting survived through rebooting.
I do have one other curious issue since upgrading to Sierra. At shutdown or restart, the iMac no longer gives the typical spinning circle it used to. I'm not sure what it's called (it's not the spinning beachball), it white on a black background that rotates clockwise and looks like the hash marks on a clock. My screen just goes blank, making it hard to determine if anything is happening until the restart or shutdown occurs.
Anyone have a clue what might cause that indicator to disappear?
