Steve,
I think most folks use 3.3uF or larger. It's not optional since the DAC in the SB is single ended it has a few volts of DC on the output so a blocking cap is required. I can say that a 3.3uF Sonicap bypassed with a VH Audio V-Cap works very well, better than the same 3.3uF cap with a Sonicap Platinum as the bypass, at least to my ears.

Value wise, it's just your standard 1st order high pass, so the cap just needs to be large enough to keep f3 low. With the SB3, there isn't enough room in the case for a large film cap like the Sonicap I'm using. The small BG NX's with a film bypass work pretty well though one could probably get a low voltage 3.3uF (or so) film cap in there but maybe not a polypropylene.
Mike