I would recommend seeing who leaves first, especially if Trump wins. The Romneys and Bushes already have one foot out the door.
A good benchmark would be what happens when Ryan gets challenged for the speakership.
If they left, and if I were a registered Republican considering leaving the party, I'd prefer to avoid leaving around the same day as they do. I'd prefer to avoid the appearance of following their example.
(I've never been a registered member of any party, but if I were a registered Republican, I wouldn't leave around now, for reasons to be explained more fully in my next post. This post is a foretaste.)