So when is the right time to lay claim to standing your ground?

And this is what we fight:

The parents of 17-year-old Trayvon Martin say Floridians who initiate confrontations should not be allowed to invoke the state’s “stand-your-ground law,” which permits using deadly force against those who pose a risk of killing or seriously injuring someone.

Pray tell, why? exactly should we not be allowed to invoke it?  Who initiated this confrontation?   It is not known, as far as I can tell.   That is why George Zimmerman sits in a jail cell in Florida awaiting trial.