As long as it's geographically convenient for your. Reason being if something screws up with the shop's service, at least it's near your place.
If you don't mind the distance (and if you're far from it), go to greenhills. DON'T go to the 'tiangge' type stores. Go to a real store. Since you got your phone for free, you could at least spend on proper service.
\m/