I haven't done the cleanse, but I don't think it would make a huge difference on your first cleanse of this kind. It's better to get cleansing that wait for all circumstances to be perfect, 'coz they never will be!!!
The amount of toxins in non-organic juice won't be significant compared to the amount of toxins in your under-functioning un-cleaned bowel, so I wouldn't worry about it.
If you were an internally pure long-time fruitarian living in an unpolluted environment you probably should make sure it's organic.