Run new brake lines if everything else is new and you want peace of mind... Whats another 30.00?
That's essentially what I'm saying as well.
If you're that worried that you think you need to clean out the lines with solvent (in other words, you're worried that they're obstructed by solid matter), then you should replace them, If not, just flush through with new fluid when you bleed the brakes.
The likely problems are: rusting through from the outside, which I've had happen a number of times (need to be replaced), having old brake fluid in them which has absorbed moisture, thus lowering its boiling point (cured by bleeding brakes), and having old brake fluid in them with so much moisture over such a long time that they are rusting through from the inside, which I've never actually seen (need to be replaced).
I don't really see a situation where a solvent flush will gain you anything, unless it's to clean out the system prior to switching to silicone-based fluid, in which case, as far as I know, it's unnecessary (I believe it's switching the other way that causes problems), and, besides, silicone fluid gives a soft pedal that I, personally, really don't like.