Looks like it is currently only used in the old postscript printer back-end, which is only used by default in wx these days if it was built on a system with a very old GTK+ library. SetPrintMode is used to control whether the printout is to go directly to the printer or is streamed to some other destination.
What other options are available to pass into it?
>>> import wx
>>> [pm for pm in dir(wx) if pm.startswith('PRINT_MODE')]
['PRINT_MODE_FILE', 'PRINT_MODE_NONE', 'PRINT_MODE_PREVIEW', 'PRINT_MODE_PRINTER', 'PRINT_MODE_STREAM']