Reportedly you can book FPs once you scan in at a park, even if you are being held at Main Street/Buena Vista Street; I follow
Disneyland Daily on facebook and she has done this multiple times.
But for RSR, those FPs don’t disappear as quickly anymore, now with GotG. Of course if you want to be on first thing in the morning you would need to get them ASAP, but if you just want them sometime that day, then you don’t need to go crazy booking them first thing in the morning!