If you select the Supply map mode, you can see where your supplies are routed through. Any spots along the main arteries with low infrastructure can be a bottleneck for operations at the far end, so upgrading a few of the worst spots can be beneficial, without spending a huge chunk of IC on it. I won't waste IC to blanket an area with upgrades, but I definitely boost some of the weak points in almost every game, even with relatively weak minors if they ever finish researching the Industrial techs to unlock Advanced Construction.
The AI does consider Infrastructure levels to some degree, and will favor provinces with at least 6 (I think that's the cutoff point) over those with less. If you create a clear path with at least 6 Infrastructure per province, the AI will tend to use it, although it may still take "shortcuts" through considerably worse provinces on occasion, and may redraw the supply route any time your units at the front shift laterally across the lines of supply. As long as your divisions move back and forth toward or away from the supply hub, not sideways, the supply lines seem to hold pretty solidly.
If you build IC (as Germany, the Soviet Union, or the United States), your Construction Practical values should be fairly decent, and upgrading Infrastructure over a moderate area or in a long line shouldn't cost 2 random limbs. If not, then you should only build it where it actually presents (or will present) a problem, because it's expensive to do on a large scale with an IC-poor country. Basically, if playing GER, infrastructure upgrades in Poland are close to mandatory. As Japan, there are about a dozen or so provinces that you can upgrade to put a decent supply highway into the Chinese interior for a significant portion of your troops, without breaking the budget. Other troops will just have to make do with the limited supply that they can get, as there are more important things to spend your IC on once the US decides to pay you a visit.
