The Knox warranty flag will only affect you if you need to send the phone to Samsung for warranty repair or replacement. If you never need to send it to Samsung then it won't matter. Sprint has really changed their rooting policies lately and don't seem to complain as much if you rooted your phone. TEP, Best Buy Ins, Square Trade, and other insurances will still replace or fix your phone, Samsung is the only one that will have a problem that the warranty flag is tripped.
Phones & Tablets: Galaxy S4, Galaxy Note 8", Evo 4g LTE, Evo 3D, Acer Iconia A500, Samsung Galaxy Tab 2 10.1, Ipad 2, Nabi 2,
Rom: Rooted Stock Odexed, Hotspot Mod, Xposed, Wanum