I'll weigh in just so Markus can have another opportunity to correct me ...
In my experience, a ground ball to an Inf will (almost) always result in a throw to first. The only difference is whether the Inf is playing in or out. If out, the runner on 3rd will always score. If in, the runner on 3rd will stay put.
There is an exception that occurs if the runner on 3rd would be able to score a walk-off run. Then he may attempt even if the Inf is playing in.
AFAIK, all this is still the case. If it has changed, I'd love to know about it.