This is always an individual decision. You have to decide what you can do with the miles instead. For example, I NEVER use miles to pay for domestic tickets. I much rather use for buying upgraded seats or free tickets internationally. Last week I used 30,000 miles to upgrade a LAX-SYD flight from business to first. Worth so much more than what I could get for those same miles domestically.
Plus, if you use points to buy the ticket, you do not earn miles.... so there is an opportunity cost there.
William