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.
I've looked to see a gap between the nipple and cap after placing it, and never seen one. Nor have I ever experienced a multiple ignition while using caps that were pinched into an elliptical shape.
In a perfect world, you'd find caps that fit nipples perfectly, but this isn't always the case.
I experienced a multiple ignition on three separate occasions with the same .44-caliber 1851 Navy back in the early 1970s. The caps were not pinched, but placed on the nipples as-is. They fit fine, leaving no gap twixt cap and nipple, but they wouldn't stay on the nipple as tightly as the pinched variety.
I believe all multiple ignitions were caused by caps falling off unnoticed during firing, or knocked off or loosened by recoil.
I was using a .451 inch ball and either Crisco or Hodgdon Spit Ball over the seated lead ball.
In later years, after reading Elmer Keith's recommendation for the cap and ball revolver, I began using greased felt wads.
Later still, I began using a slightly larger ball: .380 inch in the .36, and .454 or .457 inch in the .44s.
Since using the larger ball, greased felt wad between ball and powder, and pinched caps I have never experienced a multiple ignition.
"And lo, did I see an ugly cat. Smoke. Brimstone. Holes in parchment. And this ugly cat was much amused." --- The Prophesies of Gatodamus (1503 - 1566)