Single rooms are very much the exception rather than the rule. Do test bookings with 2 people and check that the accommodation price is identical even when breakfast etc is included.
Rather than checking to see if you are solo or a couple, the number of people field is often there to check for things like the number of children staying on cot beds etc.
I think the issues someone had in Las Vegas could have been more to do with someone miffed that they weren't getting their customary tip.