If you believe micromatic.com's site, you should clean your lines every 1-2 weeks. I have noticed no problems with cleaning them after every keg, and my kegs tend to stay on tap for around 2 months each.
If the lines are new, they presumably are "clean", ie free of dirt, dust, buildup, etc. Probably even "sanitary", ie free of bacteria. But why take a chance with either? I'd just run some cleaner (and whatever rinsing the cleaning chemical you use stipulates) followed by sanitizer, be it new lines or existing ones.
One wrinkle I've noticed is that for my taps, it wasn't enough to just keg up cleaner or sanitizer and run it through the system. One tap started getting sticky after the second 5 gal keg of beer had been dispensed. So I removed the tap from the shank, dissasembled, soaked the pieces briefly in PBW and rinsed, reassembled and reinstalled, and all was fine.