i can't say for sure but i'm going to assume that the OS is going to first give memory to all the apps you need, and afterwards, if there is any free memory left, they'll start caching it;
even if they have already used that memory for cache and you start a new app, bringing that new app's stuff in from HD is going to be a blind write onto memory, which is the same perf as you not caching it, so there really is no loss, unless there is some corner case that i am not thinking about