This is a question I had (have) as well. I think it depends on the input data (signal and confounds) and if it is already standardized.
If your data is fmriprep output, I believe it is already standardized so no more standardization is needed See also: Fmriprep pipeline and output and Denoising strategy with fMRIPrep confounds. This however concerns standardizing the signal; I am not sure about standardizing confounds.