I can't agree with this - if the termination of a transmission line is correct at each end, then the length has no matter at all for any frequency (in theory, not accounting for increasing losses with frequency, but then there's a reason for length restrictions in the CatX/Ethernet standards).
If you're talking about a *tuned* line (eg a stub or a tuned antenna feeder), then length is important. But we're not. If you've got problems with harmonics or matching and reflections then your ethernet cards are probably bottom-shelf knock-offs.
The problem with premade-lenght cables is you're going to run into tangles if many changes are made, and are going to end up coiling. Make that coil too tight and you're going to cause crosstalk. A custom job with all cables neatly following defined routes with no coils, twists or kinks is going to make life easier in the long term.