If the mis-match is on the front axle, you'll have tire scrub only when you lock the front diff. If you have a mis-match at the rear, one tire will scrub 100% of the time. That said, don't leave tires of two different heights on a real diff for a long period of time (a thousand miles?). Those mis-matched tires will be making the spider gears active full time, and diffs are designed for the spiders to come into play intermittently when you are turning. The diff will, at best, run hotter than intended.