I'm with Eric, it depends on the species. With a very mellow frog like Phyllomedusa hypochondrialis I have no problem leaving the viv wide open while I work, and will sometimes even pull the frogs out and just put them on a random plant in the frog room because I know they won't go anywhere, and on the opposite end of the spectrum I will not open my Agalychnis annae viv at night because they make a run for the border as soon as the doors crack open. In any case it is best not to leave your viv open all the time, even the calmest frogs will eventually figure out that they have an exit.