I use it on both classical and steel-string guitars. It doesn't affect the tuning noticeably in either case, though in the case of steel strings the slight force of the nylon cord on the very stiff short span of the steel strings probably does make them deflect a micron or two.

It's also true that the extra few inches of nylon cord required by this method adds to the total weight of the guitar and strap assembly, but I haven't found that to be a problem either.