Not necessary. But if you're using a MacPro, new or old, you may have issues with performance unless you optimize to prores. It's just a checkbox on import or in your prefs and it happens in the background. You can continue working utilizing the original clip until the conversion has completed.
An iMac i7 (i5 too?) has a built in h264 hardware acceleration making single pass h264 export extremely fast and appears to assist on h264 playback in the timeline as well.