You have to think in terms of fractions of a wavelength. One-eighth of an inch is negligible when the wavelength is 32' (35 Hz), but it's very significant when the wavelength is 3/4" (18 kHz). Unfortunately we don't get to operate at just one wavelength, so standing waves are almost certain to occur somewhere in the frequency range. We can take steps to reduce them, such as non-parallel surfaces, and dimensions that are not integer ratios, but we can't eliminate them entirely.
I recently built a golden ratio enclosure (width:depth:height in ratios of 0.618:1.000:1.618). The box is heavy and strong and well-damped internally, yet an impedance sweep still shows a slight anomaly exactly where a half-wavelength equals the major diagonal dimension. Its amplitude is negligible, but it's still there.