IH's tractors, except some of the "worldwide" utility models, didn't have sychronizers in the tranny. Their big tractors didn't get synchro's until the early 80's! I havent' been around a Farmall that you couldn't go from 3rd to 4th, or 4th to 5th, while rolling if you shift properly. If you're "good," you can double clutch and shift on the go. I have heard of people driving 1066's like semi's--winding them up in a lower gear, let off the throttle, ease it into the next higher gear, and hammer down again, etc. till they are in high gear. Only do that if you're REALLY good.
A little gear grinding is not good, but not the end of the world either. I try to take my time shifting to avoid it. Don't "slam" it in gear. Some of these old tractors require you to "grind em to find em" if there's a "draggy" clutch, etc. Seems like the more used to a tractor I am, the less likely I am to grind gears--if I was to climb onto one of your Cub's, I would probably grind a time or two until I got used to it's "nuances."