275/70/18 should fit but saying that, it all depends on your Jeep. If you have or plan on getting a heavy bumper causing sag. What is your current spring rate? On a Unlimited, weak springs cause sagging. And other additional weight or do you disconnect on the trail.
265/70/18 are safe. They won't rub at all
275/70/18 will not rub during full turns like 285's will but since they are taller depending on the above factors they could rub the fender during articulation. For the most part people are safe with the 275 but not knowing everything means that they could. Saying that, normal driving will see no issues at all it's just the trail that may have issue and thats just not a sure statement.