Is the usual 1/8" over 6' subfloor variance mainly for the purpose of appearance? At what point does variation in subfloor become a structural concern?

I understand that in wide open areas the reflection would reveal even minor variations. I will absolutely do the work and make those areas as flat as possible. The challenge is that I am putting wood in my entire second floor including 7 closets(due to severe allergies). Leveling just becomes time consuming given the wonderful subfloors we have. So, I'm really wondering if i can cut some corners in areas where I don't care if someone could see a little variation. However, I do need it to be sound and last more than a year.

Thanks