There can be interference problems depending on backspacing of the wheels. Some owners with 15" rims choose to grind the calipers which is not a good practice but it is not totally uncommon.
Everyone has their own preferences and tolerance for risk, but there is no practical need to run 15" rims on JK like on previous smaller Wranglers that couldn't readily accommodate larger tires. For off road performance what really matters is the ratio of tire to rim diameter. For appearance purposes there is no substitute for small rims to create the "donut" look.