I am no expert on the subject, and there is certainly a great deal of science here, but I believe antennas are tuned to a specific cable length. Check the package for a recommended cable length. The small adjustment at the end of the cable is only for variances in how/where the antenna is mounted AFAIK. It isn't intended to account for large differences in cable length.
I remember my 4' antenna called for a 15' cable but the store I was at only had a 12' cable and as a result my SWRs are higher than I'd like. I've been needing to purchase a longer/proper 15' cable but its such a low priority on my TO-DO list.
SloCrawler, I haven't read that link yet but I'm willing to bet its discussing 1/2 the distance of our radio's wavelength. I think a shorter cable is ideal, but the long lengths of our antennas (usually five+ feet) requires a longer cable?
At the very least, if you use a long cable and have left over cable length, be sure to roll up the extra length in as large of an area as possible to prevent induction (if you wrap a wire up into a coil, it will generate a small magnetic field that will affect the voltage passing through the coil). My radio is mounted only a few feet from my antenna, so what I did was I ran the extra 8~10-feet of unused cable length back-and-forth at the back of my cab under my carpet from the drivers-side-to-the-passengers-side a few times with as much straight-lenght-sections as possible to minimize any opportunities for a magnetic field to generate.