Not even one level in the US? Build more for less?
USA is probably the only country which benefits for real from PT, because they have to raise nearly an entire army. Discount will indeed save a lot of MPP's. How many levels for them are enough? Around early-mid 1941 you should start to buy units as US. So double chitting PT early game should give you enough benefit from around late 1941, as lvl 2 finishes. Going for lvl 3 will probably cause it to arrive around mid 1942, when you will be already pumping out stuff quickly. And you must be quick, you should time your landings around end of 1942, start of 1943, depending on USSR condition.
USSR will also benefit from starting chit in PT - I used to refund it, but they buyback so many units that you will feel difference in MPP's.
This isn't true for China, because they are so MPP poor, that after US aid stops for around 18 turns, they won't be able to rebuy armies, even with discount. It's better to refund the tech, buy AA gun quickly, it will pay off.
India doesn't have luxury to pick techs like this, they must prepare themselves for JP attack and you don't do this with macro techs.
UK is a strange one. Yes, you will have to buy some stuff, so discount would pay off. But really Churchill has to be irritating for GER from the get go, and you don't do this by investing in macro techs.
That's my personal opinion. You may argue that in mathematical terms PT nearly always pays off MPP wise. But this doesn't take into account, that it's 1941-1942 period, which determines the outcome, at least against very good players. And I like to asses situation as if you were playing against World Champion in this game, this forces you to make meaningful decisions every time.