I think you have to go with what you are happy with, really. It is you that is stood looking at your horses, and you that knows them, so if you think that they would be OK turned out, go for it! I bought a colt in the summer who had been on what I suppose you would call a dry lot- it was a row of very nice pens with run ins, and they were in a separated herd environment- each had it's own pen. There was a (very lovely) stallion in there, mares, two foals, really nice, but NO grass, zilch, concrete base that was washed down every day or so. I just turned him out with my boys, never gave it a moments though to be honest, and this was four months ago. He went form being in all the time to being out 24/7 with five strangers and no pens! He coped. Of course I watched him carefully, but he was fine. Another horse, different metabolism, maybe a bit overweight or a filly (not so boisterous) I would possibly put a muzzle on, I don't know- I have only just come round to looking kindly on muzzles, so probably not. I am a great advocate of hot wire strip grazing, though. Whatever you do decide, if you turn them out for a couple of hours then bring them back onto the dry lot you could do a lot of harm- you just get a bunch of little horses trying to shove a day and a nights worth of grass down their necks in a couple of hours, which in my book is a recipe for disaster!! Good Luck.