I've always trusted the stylist I've had and I can say that for me it's because I really get nothing dangerous done to my hair :-D  It's a simple trim off the bottom, maybe a bit of an angle.  I don't dye it, don't perm it, don't do anything like that, so I assume the hairdresser can't really mess that up!

Granted, I like to try and stick with a hairdresser over time, but I really go so rarely that I'm like a new customer to her everytime I go!