Depends on what you hook up to each HDMI. I assume this "setting" is also for EACH HDMI...not a "blanket for all" HDMI setting.
Most BD players are set up for -10db from the factory to take into consideration you "will probably send" all the other 5.0 sound to the .1 under 80hz...so all those "other" 5.0 under 80hz will each add 2db...
2db x 5.0 = 10db...hence why BD players come -10db.
That setting is probably to compensate for the low suwoofer output of the Multi-Ch inputs when using DVD-A and SACD. I'd leave it set to 0. Use the adjustment in the speaker setup menu to calibrate the subwoofer volume to the other speakers. Some receivers/pre-pro's have a -10 setting to lower the subwoofer output when using a two ch analog connection.
But it wouldn't harm anything to set it to +10 if it sounds better for some reason.