If I am not buying from my PM then is it worth teching/buying mil tech just to keep military expenses down? Or would it not be better instead to tech/buy bus & res tech?

Depends on how much military you keep onhand.

yo nerrus it's definitely worth it, and it's really worth a private or hybrid private pub jump at the end if you make it that far and don't always go to battle like me :p
expenses add up over the set and it's a great tech that sells all set long AND it is the best selling tech in the last 6-18 hrs of the game

but you want to lead with bus and res, max that early and always tech it first and along with mil, and then tech anything else after those turns, like, weapons, strat, indy
i think you get the idea there

in the first place why are you not buying military from your private market?

Cheaper to buy military units from the market instead; only time I get Military tech is when I think I need to do end of round stock up from the the private market.

it wouldn't be cheaper if you teched mil though :p

I have always built up a stockpile of Military when playing techer but I have been cutting back on that of late. The saving in expenses is miserly compared to the gains in income from Bus and Res.

That said towards the end of the set it becomes a very good seller when other techs (including Bus/Res have gone way down and you get the substantial plus of the cheap units.

On the whole though I have been starting to think that the answer to your question is that it is not worth it.