Vegas is probably dry enough where attic might work, but it is not a good idea nor is going out the soffit for most of the country. The best locations for the vent are roof, gable end wall, or through the wall to the outside. While many people end up going out the soffit, it may not end up being much different than into the attic since in many configurations, there are roof vents that are sucking air in right next to that outlet. The issue with going into the attic is it can dampen the insulation on a cool day because of condensation. It could damage the wood as well. This is more a problem in humid locations. For most of the year, other than the perfumes and other smells, the added moisture in the house might be good rather than venting to the outside. Personally, I have a problem when the humidity gets into the teens and single digits.