I have a non-sideband version already that I'm using with my Adaptec 31605 and 4 WD Raptors without any issues but I needed a second cable for some additional drives I've yet to install and no one in Canada had stock of the non-sideband version so I ended up ordering one with.
From what I've been able to find on the internet the sideband is an additional bank of connection on the same socket that aren't generally part of the SAS standard and are used by various vendors for additional signally, between a controller and backplane for instance. I'm guessing that the additional WD Raptors won't have the associated connections and will simply ignore the presence of the sideband altogether.
Considering the retail version of the 31605 comes with a sideband cable I figured I'd be safe. (I have the OEM version that came with no cable at all)
I'll try calling Adaptec on Monday to find out.