While poking around in advance of changing out my rotors and pads, I
noticed that I have some rust on some of my hard braked lines.  So now I
figure I should change those too.

So far I have only seen rust on the rear lines, where access doesn't seem
to bad.

I'm assuming the worst lines to change are the ones that actually go to the
master cylinder.

Any thoughts on sticking with a partial replacement?  Apart from obviously
being less work, this means a lot fewer disturbed fittings.