Physical layer impairments may require highly complex implementations of the control plane in dynamic networks with Quality of Service (QoS) constraints. In order to alleviate this problem, we propose that dynamic wavelength assignment should be based on the elementary First-Fit (FF) algorithm using off-line optimized priority lists. It is shown that this may significantly reduce the effects of physical impairments with no additional computational cost for the control of dynamic networks. A simple metric is proposed to evaluate optical QoS taking the performance of the network as a whole. Different heuristics are presented and evaluated. Results show that there are particular lists capable of reducing in four orders of magnitude the probability of FWM impacting network performance.