I don't think there is any need to replace the hard lines, unless there is damage, corrosion, or you just want new shiny ones.
I used to believe that, even after I had an accident that I attributed to a hard line rusting through (it had surface damage that I did not know was there).
Then when I was putting TS13571L together in 2008, I went to my bin of old brake parts and tried to pull out one of the links between the reservoir & MC. It was caught on something, so I pulled a bit harder, expecting to lift the line and whatever it was caught on (probably a mounting bracket). Instead, the line literally snapped in my hands! Examination showed no signs of corrosion or previous damage, and the steel line was not deformed (proving that I hadn't pulled very hard on it). It just broke!
Now obviously, that section doesn't see any pressure in operation. But it appeared to be the same construction (basically solder-coated seamless steel tubing) as the other brake lines. So if it broke, the lines that do see pressure (as much as 1000 psi in a panic stop) might do so as well. Not a fun thing to lose your brakes, as I've learned several times now. And even a properly working handbrake is surprisingly ineffective at stopping the car quickly.
I promptly ordered a set of Cunifer lines from Moss (made by Automec in the UK). To be honest, I did not finish installing them as I was in a hurry to get the car finished in time to drive to VTR 2009. But the sections I did install went in easily. The Cunifer bends much easier than steel does, although it is still a good idea to use a mandrel to avoid kinking the line. I still plan to install the rest of the set, but there are some higher priority things on my plate (like repairing the damage from the pickup truck that "didn't see me").
One other lesson learned : The Cunifer abrades easily, without making the kind of noise I'd expect from steel. So be sure the jumpers on the calipers are neatly tucked down where the wheel cannot rub on them. This little mistake caused a total failure of the hydraulic brakes (fortunately just as I came to a stop in a parking lot).