You don't HAVE to reflash as it won't hurt anything, but you will get the CEL. Thats because when you install it, you remove the Cats and the Cat sensors. Doing so makes the computer think something is wrong with the cats and thus the error. I drove my O/R H-Pipe for about a week without any problems (other than the CEL) before my tune came in.
I got mine re-dyno tuned when I put on an O/R Xpipe..Just to make sure it wasnt going to run too rich, and the fact that I like to be safe rather than sorry...