It depends. Who refurbished the equipment? If it is the original manufacturer, then it is probably okay. It could be a unit that was purchased and then returned because of some defect (real or imagined), and then it was checked out and repaired (if needed) by the manufacturer. If that is the source, then, if you don't mind that someone else touched it, it should be fine, though, typically, warranty periods are shorter for refurbished items. As for demo equipment, it depends on how it was used/abused. Other than typically getting a warranty, it is like buying used. I have owned both types of equipment (though not A/V receivers), and have had no more troubles with such equipment than other equipment I have purchased new or used.
I have heard about some great deals on Yamaha refurbished receivers via ubid recently, and if I had been in the market for an A/V receiver, I would have purchased one.