Stress relaxation in NiTi wires directly affects their potential for actuator applications. Here, we report on simple stress relaxation experiments on fully martensitic wires at room temperature after loading to different total strains from the small strain range (self-accomodated martensite) up to relatively large strains (plastic deformation of reoriented martensite). Interestingly, the strain rate applied prior to stress relaxation has a distinct effect on the subsequent relaxation behavior even though detwinning is an athermal (and hence rate-independent) process. To rationalize our findings, we discuss the interaction of dislocations with twin boundaries as a possible key mechanism for stress relaxation in NiTi. Stress drops at constant strain are analyzed to determine experimental strain rate sensitivity (ESSR) values. When evaluated in Haasen-like plots, the ESSR-values exhibit a shift towards higher stress-levels in the deformation range where detwinning and martensite reorientation are most pronounced. This indicates that, from the point of view of dislocations, detwinning substantially alters the microstructure in terms of twin boundary spacing and the mean free path available for dislocation slip-based relaxation processes.