Well, assuming that the selected shutter speed is valid at 60fps, off the top of my head I see no reason why that same shutter speed cannot be mantained across the entire ramp. The maximum possible frame rate of 60fps has the tightest open-shutter time restrictions, obviously because the frames are closer to each other in time. However, if you have a valid shutter time setting at 60fps, ramping to/from lower frame rates is not an issue with exposure because it allows for even more slack time between frames which permits that exact same shutter time.
Cheers,
Juan
Cheers,
Juan