I don't find any potential danger in pinching the cap into an elliptical shape. Been doing it with my fingers for years.
Though the cap may be elliptical, once placed on the nipple it returns to its round shape. However, it retains some tension from being out-of-round and clings better to the cap.
thank you - I was assuming that the way to do it was to place cap on nipple and then pinch tighter . . . I'm using #11 caps as I cannot find anyone locally with #10's (must try harder) - shooting the gun is good fun though!