I agree that percentiles are very useful in general, for instance when 
you're dealing with independent components maps etc.
However, I tend to disagree when it's about a z map that comes from a 
glm: the common use in the community is to threshold according a given 
type I error control. The defaults thresholds adpted e.g. in SPM are
- p < 0.001 uncorrected (z >3.09).
- or p < 0.05 corrected (z>4.8 approximately if you use 3mm voxels).
I think that these are reasonable  rules -- as long as we're dealing 
with a z map.

Sorry for bringing such a mess in a quite unrelated PR.