it basically depends on your frequencies as well as on the length of your traces.
As a rule of thumb, if the trace length is shorter than a tenth of the wavelengt of the highest frequencies (\[{l}_{trace,max}\] < \[\frac{{\lambda}_{max}}{10}\]), wave propagation effects are negligible.
The PCB structures have to be analysed as planar transmission lines, line type according to the geometry e.g. micro strip, embedded stripline, coplanar strips with or w/o ground, coplanar waveguide.
@wwfeldman
It means smaller than a tenth. So if the length of your trace is shorter than \[\frac{\lambda}{ 10}\] (e.g. \[{l}_{trace,max}\] = \[\frac{\lambda}{12.3456789}\]) wave propagation effects can be neglected, see The RF and Microwave Handbook.