V Rai, if I remember correctly, the ohm settings/switch on Yamaha receivers are placed there simply to satisfy UL requirements/for liability reasons, i.e. that switch reduces overall power output but prevents overload/overheating.
It is recommended that you keep the settings to 8ohm to regardless of your speakers impedance in order to get the best performance, just make sure the receiver is well ventilated.
More on the topic: http://www.audioholics.com/techtips/buyingguides/avhardware/receivers.php
BR,
Sany.