How can you save specific instruments with midi tracks so they don't have to be reassigned each time the file is opened up?

Which synth?
For many GM synths this happens automatically.
For some synths, inserting a patch change at the beginning of the file and saving it once works better, and yet others (for some reason) Save Group seems to work better.

But there are some that simply have to be reset due to how the synth interacts with RB.
Explain your MIDI set up and we can offer more concrete suggestions.

Heck, the Track Category feature mentioned in a post a few below this one may even help, since it does allow setting up a virtual Save Group scenario that loads when the song loads.

Thanks for the reply. I am using the the rerouted one - the Dxi/VSTi.. But I was using the fx button to select instruments instead of right clicking on the midi icon and selecting program. And I notice the options for instruments are not exactly the same. There were 2 electric pianos with the fx button but only one the way that will save.

Select the instruments track by track, that is most reliable for basic use.
Right-click the track and go to Programs, then follow the desired path for the desired patch. This works fine for GM sounds.
Patch Select dialogue also works, and can be more customized if you have the synths to suppoert it.

Inserting a program change (patch change) Event on the track itself would be next in my experience.

Coyote synth?