Hmmm, crazy how different techniques can be. I've done flagstone set in sand and gravel. I prefer sand for around here without any problems to date and some are 10 years old. To be fair though those patio's had very fine mulch for joints so maybe they help fill a void if they appear. I want a base that is going to drain so I don't have water sitting and freezing under our stone creating popping and flaking. That's why I have been doing permeable flagstone joints and base on the last couple jobs. I'll spend an extra hour or two gauging my own flagstone in piles of thickness to make installs a little quicker.