Inherent to time-domain method that use space-time discretization, the model dispersion produces field propagation velocity error. In methods such as finite-differences or TLM in time domain, it is customary to choose a maximum mesh size of one tenth of the minimum operating wavelength. If this criterion is easily applicable for normal ordinary media, it is more ambiguous to give interpretation of the wavelength when dealing with complex media such as anisotropic and/or dispersive (frequency dependent) media. After describing the procedure to evaluate dispersion in such media, the paper shows examples for which the simple rule underestimates the maximum mesh size and, consequently, increases the computing expenditure.