[silverstreak]

I only like to replace what is necessary, since the factory steel is usually thicker and the bends are usually more crisp. I have replaced small sections like that before in the rockers of a Chevelle years ago which worked out well.
Here's my advise, cut the bad area out that you intend on replacing (1x3) then get one of those small pivoting mirrors and a light to look at the inner surfaces of the rocker. Make your decision on weather the patch will suffice or if you need to extend the repair further or even replace the whole rocker, based on what the mirror reveals. Rust is usually worse on the inner surfaces since the outer areas are better protected with paint and regular (sometimes) washing.