Most head positions don't really put mush more stress on the neck wires. The wire are always kept taut and use a pulley system. Left straight, or right really don't make any difference. Up and down have stress related to the weight of the head. If the head is resting on something, that relieves the stress. Again, up, straight, or down don't make that much difference as long as the head is supported so that the wire isn't holding all the weight. Use a pillow (or a stack of books) to take the strain off the neck. So sleeping position doesn't cause the wires any more problem than the packing position.
According to Innvo, the 2009 pleos did NOT use a heavier gauge wire, they fixed joint/servo/pulley problems that were causing friction/rubbing on the cables which led to cables breaking from excessive wear. (You rub a shoe lace across a brick long enough, the shoe lace will break. The problem is not the shoe lace, it's the brick...)
All that said, the sleep position IS BAD for storing pleo when turned off. Not because of cable issues, but the SKIN!! The sleep position stretches the skin in the neck and tail, that over time may lead to growing rips and tears in the thin spots of the skin.
The best position for storing pleo from the persective of the skin is the packing position. This eliminates most (if not all) of the stretching of the skin.