Spend like crazy!
1. Sometimes, you need to go pure defense with Russsia. When doing so, if you have 17 IPC, you spend 15 to get 5 infantry, and save 2 IPC. Next turn, with another 17 IPC income, you will have 19 IPC, allowing you to get 6 infantry.
2. Sometimes, you want to be able to switch fronts with the US. So you don’t spend a lot first turn, then second turn, you commit to either Atlantic or Pacific. (If you do spend in the Atlantic on US1, it takes TWO turns to sail to the Pacific, and vice versa).
In general, it is not a good idea to save.
UNLESS UR JESUS!