Many folks raise their bars 1" without requiring lengthened cables. I bought a pair of 1" bar risers (Motorcycle Larry, a brand not available anymore) and was not comfortable with the tug on the clutch cable. So I purchased a clutch cable for a Honda Transalp, 2" longer than the NT's, for about $24 I think, Honda part #22870-MAB-620. I haven't installed it yet.
Others can chime in on their riser preferences.
I bookmarked this thread and may go with different handlebars instead of risers:
http://www.nt-owners.org/forums/showthread.php?11884-Handlebar-Option!
Raising the bars here is common, and using the search function will yield you quite a bit of info.