Yes that is correct, once you have used REX to replace certain textures with ones that you have decided to use from REX then you no longer need to have it running. The other good point of this software is that you only have to change particular textures using the Options page. You deselect any texture that you don't need to change and select the ones you would like to change. So if you want to change the concrete runways only, you select that only in the options page.
E8400 @ 3.0GHz, DDR2 Ram 800MHz 4Gb, Vista Home Premium 64Bit, MSI 9800GT @ 512Mb, Philips 22" LCD Widescreen, Cockpit Setup X 5 using Wideview and Widetraffic.