JKs are unlike a lot of past jeeps where you needed to long arm them to get a lot of flex. I have the full set of teraflex adjustable control arms and although i wont be setting an RTI record, it flexes great for my uses.
Control arms are an expensive upgrade and I would say you are generally ok to run any lift up to 2.5, maybe 3 inches without having to upgrade the arms. The main issue when you lift the jeep and keep the stock arms is that it may feel a bit flighty. Adjustable short arms help to solve the issue as well as get your pinion angle correct to alleviate driveline vibrations or other related issues. Long arms on the JK will provide a bit more flex and will certainly make the ride more stable as well. Some of the newer kits for the JK have long arms tied in from new cross members which also help to strengthen some of the notoriously weak spots underneath.
There's a lot out there about control arms, but I hope this puts it into real-world terms for you...
1972 Commando on Tons & 40s