With the widespread availability of SPECT/CT systems it has become feasible to incorporate prior knowledge about anatomical boundaries into the SPECT reconstruction process, thus improving observer performance on tasks of clinical interest. We determine the optimal anatomical-prior strength for lesion search by measuring area under the LROC curve using human observers. We conclude that prior strength should be chosen assuming that only organ boundaries are available, even if lesion boundaries will also be known some of the time. We also test whether or not the presence of anatomical priors affects the observer's strategy, and conclude that mixing images with and without priors does not hurt reader performance when priors are not available. Finally, we examine whether using an anatomical prior in SPECT reconstruction helps observer performance when the observer already knows the possible lesion location, and conclude for this task anatomical priors do not provide the same improvement seen in search tasks.