Think about this... round nipples, round caps = pretty good seal against sparks.
Round nipples, oblong caps from being pinched = ???
If the caps are the right size, you shouldn't need to pinch them and if they are the wrong size, you've created an even larger gap between the cap wall and the nipple, though they are less likely to fall off, which is the absolute worst.
Caps, (at least Remingtons), are all the same diameter. What's different is the height of the caps (length of the walls). Nipples are tapered, and you want your cap to start making good contact very slightly before it bottoms out against the top (back?) of the nipple, becoming fully seated.
If it makes this contact too soon, you will think the cap is seated all the way when it's not quite, like having a primer not fully seated, only impossible to see.
And like a primer that's not fully seated, a hard enough strike may seat it the rest of the way and set it off. The hammer can hit it a lot harder than you'd dare to while loading, even with a wooden dowel to help (unless you love trouble).
Most likely, though the first strike will seat it, and the second strike will fire it (or it may never fire), depending on how well the first blow actually seated it. That's what happens when you use #10 caps on nipples that were designed for #11.
A #10 cap has longer walls than a #11, so it expects the skirt to start making good contact closer to the chamber.
If you put a #11 cap on a nipple with a taper designed for #10, the cap will bottom out at the top before the skirt has made a good seal with the tapered area of the nipple, and you'll have trouble keeping it on, in addition to a slightly higher chance of a chainfire.
Chainfires that happen despite properly-sized caps and balls are so seldom as to almost be an urban legend, but they do happen, due to defective chambers. If a chamber was bored oversize or out of round, you CAN have a risk of chainfires, despite using all the correct sized loading supplies.
THAT is when a lubed wad over the powder can do you more good than simply reducing fouling.
I think that Italian revolvers of more recent manufacture are a lot less likely to have chambers that are out of round, though. QA seems to have come a long, long way in the 21st Century all the way around with Pietta & Uberti both.