Hi, I'd say that ID is nice to have on guards, but not essential. And it's hard to train. Here is some more detail:
1. Many guards get 1vs1 training, which grows their inside shot. Having guards that have strong ID helps counter that.
2. However, guards are usually short and training ID on short players can be painfully slow. A 18yo 6'0" player will pop about once per three training events.
3. ID does grow as a modest side effect of Pressure training (which primarily trains OD).
4. I don't think ID has a strong effect on guard salary. However, if a player's ID grows very high, they might become classified as small forward, and in that case their salary is more effected.
What I normally do is try to buy guard trainees that start out with relatively high ID (say respectable or strong: I even just got a 19yo that started out with proficient, and in next season's batch of draftees there might even be guys that start with prominent ID) and then grow them via the side pops they get from Pressure training.
Hit me up in PM if you want for more questions -- I'm always happy to help USA managers and grow the USA community. (And well check out the discord if you're interested).