The 3.21 gears are fine on our automatic 2013 Sahara - getting between 21 and 22mpg on the highway with cruise around 70mph. We do plan on lifting and putting on bigger tires though so see 4.56 gears in our future. To summarize - 3.21 is fine for stock tires but any increase in tire size will warrant deeper gears to keep you happy.
Current - 13 JKU, 01 TJ
Gone - 03 TJ, 01 XJ, 00 TJ, 93 XJ, 87 XJ, 87 XJ, 53 CJ-3B