I tend to use the $300 threshold for domestic trips, but I always prefer to gift trips to others, and pay for myself in order to line up on elite status. Still, with awards not always available, I look for opportunities to burn miles. I won't get into a discussion of which airlines have better availability, but you can't count on a free flight on a certain route on a certain day. Plus, I don't want to build up
a big balance on any carrier (more than say, 200,000 miles) since it is hard to tell who may or may not be flying a few years from now, not to mention the possibility of devaluing miles.