I know my bags fit in the United sizers
Then there should be no concern of ½ an inch. Unless of course you are already just barely within the limits and you add a few things. When I travel, my bag is nowhere near the limit, so I have no concern. I don't count on some gate worker cutting me a break. What I do worry about is that, because he cut so many people in front of me a break, the overhead bins will be full of "only slightly over" bags by the time I get on board that I will have to check my well-within-limits bag. If an airline enforced their rules and made it public that the rules will be enforced, I would probably fly them.
I have a few suggestions. First is that the airline stop gate checking oversize bag for free. Free gate checking just encourages passengers to try to carry-on over-sized bags. At the worst they will gate check your bag, and you will have to pick it up at the carousel, but you'll always save the check fee. Start charging more to gate check over-sized carry-on bags than to check them at the counter. Place sizers at the check-in counter. If your bag fits at the counter, it will fit at the gate (unless, of course, you added things before you got to the gate). Then if their bag doesn't fit at the gate, the airline gate checks it for $10 more than the counter check fee.
Or even better, make them go back to the ticket counter, where they really should have checked the bag in the first place (no gate check) and, of course, probably miss their flight. To bad, you should have checked it at the counter or made sure at the counter that is fit the sizer.
Ok, not as drastic, since the bins are big enough to fit a 22 x 14 x 9 carryon, wheels to the back and 14 inch width vertical in the bin, you only need 9 inches of width in the bin. Since the seat pitch is at least 31 inches and there are three seats from the isle to the window, put vertical dividers, dividing the bin into 9 or 10 inch wide sections, with a seat number assigned to each bin section. So each seat gets an assigned 22 x 14 x 9 inch space for their stuff. If their bag fits in the space, no problem. If it doesn't, you gate check it for a fee more than regular counter check. If you bag doesn't take the entire space, you can put in your jacket, laptop, whatever. If you don't use the space, you can auction it off to the highest bidder.