Do I need to ‘GNSS Enable’ my geodatabase and map document (MXD) for ArcPad-only workflows?

No. GNSS enabling (the creation of the TrimbleSessions feature class and layer) is only required for the ArcGIS for Windows Mobile workflow.  One caveat here however…by GNSS enabling your MXD you are ensuring that a transformation is selected in the case where your data collection layers are exclusively non-WGS84.  The TrimbleSessions layer will always be in WGS84 and thus adding it to your MXD (through GNSS enabling) will prompt you to select a transformation.  So if you are using the ArcPad workflow and elect not to GNSS enable your MXD, you will need to use the ‘Datum Transformations’ functionality within the Administration window in order to define a transformation from WGS84 to your working coordinate system.