Does head size really matter?

In some guns/barrels, yes the head size matters. I had a .22 Marauder that would shoot one hole groups at 35 yrds with 5.55-5.57 mm head sizes. The brand and shape of the pellet didn’t matter. Anything 5.52 and below would not hold a 5” group at the same distance. It was my first gun and I didn’t know much about it until I bought a PelletGuage. I was using a tin of crosman premiers from Walmart and I started measuring. Wouldn’t you know I had almost 50/50 split between 5.56 and 5.52 mm heads. I’m sure all the dies in the factory spit out the pellets onto a conveyor belt and they all get mixed and put into the tins. 
So in my findings, the “flyers” were the 5.52mm head sized pellets. I was chasing an impossible expectation of accuracy. I found that the tin of JSB 18.1 grain had 98% 5.55mm. It shot them so well! I also didn’t have to spend time sorting the Crosmans. People complain about the higher price but for me it was a matter of, “Is it really cheaper to buy a tin for half price when you can only use half of them?” No. 
Hope this helps and if you can find one that is not pellet (head size) picky, hold onto it with both hands and don’t get rid of it, unless you want to sell it to me that is

Yes I believe it does from my experience. Understand though when you buy pellets by head size that doesn't mean that's what you wind up with. In general though I find pellets that fit snug (not tight) over pellets that fit loose tend to shoot better.

+! for T E great explanation P H

Glad to see more folks are finally waking up to this fact that "head size DOSE matter" know if we can just get the Mfg's to put in the tin what is claimed to be in there!! Wishful thinking!! J.L.