I wrote 3dclust back in the Paleolithic days of AFNI, and my decision then to report only 1 decimal place was for 2 reasons:
- Keeping the table compact
- Not pretending to have more precision than really exists
About the latter point: even printing to 0.1 mm accuracy is really too much -- considering FMRI voxels are usually 2-3 mm in size. I can't see how it's reasonable to expect precision to the 10 or even 1 micron level from such data.
Only for a special application to super-resolution voxels (probably not from MRI) would it be reasonable (IMHO) to give more decimal places.