[Freesurfer] Advantage of having skull for longitudinal processing?

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Hi Freesurfer team,

I'm thinking of pre-skullstrip'ing my brains prior to being input into FreeSurfer.

My fear was this information in the skull might be useful during mri_robust_register or mri_robust_template for longitudinal analyses:

Martin discusses this in his Freesurfer workshop talk at 9 minutes in (youtube: http://www.youtube.com/watch?v=vxJODPBLd3M&t=9m21s) where the heatmap covers parts of the skull and the Freesurfer wiki which mentions that the Tukey biweight will deweight regions of noise like tongue/eye/neck differences.

But it appears the longitudinal stream registers everything on each cross-sectional's norm.mgz to create the fused norm -- which is skullstripped and missing neck/tongue/eyes/skull already.

Am I missing something? Is it alright if my T1.mgz, rawavg.mgz, orig.mgz, etc. do not have a skull for longitudinal processing?

Thanks, Adam