what do you mean by open? you mean giving them access? if thats the case...I would wait until you see an egg appear-then put the box in with some golf balls in it or fake eggs-seems to work when my friend does it-mine were in their nesting boxes for a good month or two before they layed eggs in it-they never messed in it-they used the other side of the coop for that or their food bowl!!!!
Thanks to the advice I received on this site, I pulled the nest boxes out of my coop. My hens were roosting in one of the boxes. If and when I see any significant behavior, or an actual egg in the coop, I'll put them back in....with some golf balls!