When to install GFCI outlets?
GFCIs (in proper operation) interrupt the flow of current when a ground fault is detected.
That is, if the amount of current flowing out of one of the sides of the socket does not exactly equal the amount of current flowing back in the other, it'll cut off power so that no electricity flows out of the outlet.  (Some configurations protect "downstream" outlets as well.)

The possible safety benefit comes into play only when something is actually plugged in to a protected outlet, and the real safety benefit is realized when (a) something is plugged in AND (b) whatever that is has a ground fault that would otherwise put someone or some property in serious danger.   

The other slight possible benefit of replacing the outlets before use is the possibility of discovering some existing dangerous wiring issue if it exists in the current outlets, but the odds of that are relatively low and the odds of you putting such an error in (especially if you do it yourselves!) seem a whole lot higher.  If you're concerned about the existing wiring to outlets you aren't using, you can probably just turn off the circuit breaker to that circuit, so there won't be electricity in the outlets.  This assumes you're not using anything else on the same circuit.

If you're not using those outlets, there's no benefit to replacing them right away, and there is of course a cost to doing so.  You could buy replacements and wait on installation until some time when you have an electrician over to the house for something else, when the marginal cost of replacement is minimal.  Then you can benefit from GFCI protection if/when you do start using those outlets, and (assuming they still work) have one less item on the next buyer's inspection report to help that buyer feel more at ease.