Typically, it is zero. However, it is possible that certain voices have a different default. All you need to do is just call up the particular voice in question by selecting the VOICE function (with the buttons in the group that also has SONG, STYLE, and DJ/GROOVE pattern), and then use the numeric keypad or data wheel to select the voice you want to check. When you do that, it selects the main voice, and it will automatically come up with its default settings, and you can just check what they are. You don't want to call up the voice by way of registration presets (assuming that you saved the voice in question to a registration), because if you had previously made any changes to the octave setting (or any other voice function), then those changes would be part of that registration, and you would not necessarily be looking at the default value.
EDIT: I just checked some of the voices on my PSR-E433. It looks like most of the voices default at an octave setting of zero, however, a lot of the bass voices default at -2. But I didn't check every voice, so some of the other voices may default at other settings. Like I said -- just best to check the settings on the voice you want to use.
