Good question. Based on your
platinum tag I assume you're playing Generation 4? If so, then yes what you've heard is correct. In every game up until Generation 5 (Black/White), EVs are "revised" and recalculated each time the Pokemon is entered into the PC, as well as when the Pokemon levels up. So if you have a level 100 Pokemon with incomplete EVs, you can deposit and withdraw it from the box and its stats will be updated if it gained EVs. The reason you're not seeing any changes might be the way in which your Pokemon (supposedly) gained EVs; in Generation 3 and 4 only, Pokemon will never gain EVs from a battle they get no experience points from (ie. at level 100), so you can only gain new EVs from Vitamin items. Definitely try to get your Pokemon's EVs organised before they close in on level 100, or you might have to spend a ton on Vitamins.
As of Generation 5, EVs simply update after each battle, and Pokemon gain EVs regardless of their level or whether or not they get experience points, which means you can max out your Pokemon stress-free. Generation 6 simplifies it again and makes it so any changes to EVs apply immediately -- it can be safely assumed that this has remained unchanged in Generation 7.