Ich doesn't technically 'lay dormant' in the sand. The cyst stage (Tomites) will be in the sand. But for there to be cysts, that means they had to have been on a host just prior to forming the cyst. When the cysts hatch (typically 5-8 days after formation, but can be up to 72 days) they will have to find a host or die. Even the healthy fish are then carriers. But, the healthier the fish is, the less likely Ich is to overwhelm the fish. However, Ich will still be on the fish somewhere, typically the gills.
A stressed fish will have a weaker immune system that will allow the parasites easier access under the skin, and thus will/can be quickly overwhelmed by the next bombardment of hatching tomites. You would see the effects (the white spots) once that stage jumps off the fish.
Regardless, yes, keeping fish healthy and a less stressed as possible will greatly increase your chances of not having a fish die due to Ich. The point I always make though is why not give them the comfort of having NO parasites at all...? I can't imagine it being comfortable for them to even have a few parasites chewing on their gills.