If routine ground ball to an infielder, they come home and runner at home is safe and you believe it would've been an out with ordinary effort had they gone to first, then FC with RBI (and a big handshake for doing the job).
I would always score fielder's choice unless in your opinion the batter would've been safe at first if the attempt had been made at first base and no out was made on a preceeding runner.
An example is a runner at second and ground ball deep in the SS hole. SS makes backhanded diving stop and has no chance (in your opinion) of getting batter at first, so makes an attempt on runner at third. If runner at third is out, score FC. If runner at third is safe, score single. If runner at second holds and fielder flips to third baseman, single.
Another classic is the runner at first, less than two outs and the pop up to shallow center. Fielder's converge but ball drops and they throw to second for the out. In this case FC. If runner at second is safe, I score single. Even though they made an attempt on the preceeding runner, the batter would've been safe if they'd made the attempt on him. He's usually rounding first wondering what the heck the lead runner was thinking.
I admit I'm having trouble matching this all up with the rules, but it's the way its always scored.