[Figure from Eur. J. Neurosci, doi: 10.1111/ejn.13717]
Q: When building ROI-level functional brain networks from fMRI data, should I apply spatial smoothing to the voxel time series?
A: No you should not, what were you thinking? See above; it messes up your degrees and links non-uniformly, and in general has weird effects. In any case, you already average your voxel time series to get your ROIs, which is brutal enough. For more, see our recent (open-access) paper in the European Journal of Neuroscience, with @TuomasAlakorkko and @eglerean and @hpsaarimaki and Onerva Korhonen.