Dry spots can be a sign of too much pressure. That will eventually lead to white patches of hair which is scar tissue. If ridden in a saddle like that for too long the horse will eventually have issues. You want a nice even sweat pattern, no dry marks. That shows you the pressure was distributed equally.
On my warmblood when I first tried out the saddle I have no, I rode him long enough to make him sweat and when I took the saddle off at first glance it looked like he had a dry patch of hair near his withers, but once I felt it, it was just as wet as the other parts of him--his coat just looked different. Of course I rode him in it again to make absolutely sure. He's a very honest horse and will tell me when things are wrong but he never fussed about the saddle and he moved so well in it. So I bought it! lol. I KNOW it fits him, however each time I get off I always check to make sure he has an even sweat pattern, just to make sure things are still lookin good.....I'm paranoid I guess lol.