Using FP for Shows

Can someone explain how best to use FP for shows? I’ve read over and over that FP isn’t needed for shows, but @seebee’s trip report mentioned grabbing a show FP because they’d be cutting it close and wouldn’t have time to wait in line. How close to the show do they typically open the standby line? We’re taking our DS2, so we’re trying to minimize lines and waiting as much as possible, because he’s a wiggler! (We’re going at the end of January, so the crowds should be low.)

When we had a FP for FotLK, we were actually told we had arrived too late to use it because standby were already going in. They did take pity on us and let us in though. We had seen the Nemo show and then rushed from there, I would say we arrived 10 mins before the show.

I grabbed the FP mainly because I’ve hear that Frozen sometimes fills up to capacity, and we didn’t have time to wait in line and be near the end of the line, and possibly not get in.