If I’m in a fast draft on a flat road, I’m usually already in the right gear so I just spin up in whatever gear I’m in.
If I’m cresting on a climb and need to finish on a sprint, I shift down 2 and spin up. When climbing I produce FTP efficiently around 50~55 rpm, obviously at higher torque than what I’m doing on flats… so coming out of that sustained FTP scenario it’s easier for me to downshift first if the situation calls for producing and maintaining peak power for another 15~30 seconds.