>Is it in a main.prg, or in the init event of a form, or at the beginning of a program?

If they're declared before they're needed, PRIVATE scoping works. There's a strong implication that something prevents declaration at the top of the app.

>Do I have to release all those variables after using them?

If you want them to go away, yes; otherwise, they stay in VFP closes.

>What happens if I don't?

They remain until you close VFP; side effects may present unexplained logic errors.

>I've seen some posts of people not wanting to use public variables. What's wrong with PV's?
>

They break encapsulation, so that the scope of influence of a routine that changes or examines a PUBLIC is not well-defined. In many cases, a PUBLIC really scopes to the life of an object rather than the whole app; you can use a member property instead of a PUBLIC to. Declaring them as PRIVATE at the proper level in the app forces you to examine the scope of things, resulting in a better understanding of the task at hand.