Looking at the picture you posted the plant looks healthy in every other respect. The leaf curl is a small issue that I would keep an eye on but not panic over. I have some strains that seem to do this more than others. I believe that the leaves are trying to 'protect' themselves from either loosing moisture or from temps by curling. Thats just my opinion though. From what I have noticed in mine is that it seems to occur in the sativa dominant strains more than the indicas. Probebly due to the narrower leaf structure being more sensitive to drying out. Again, just my opinion.
If it gets really bad, then look at such things as room temperatures, especially hot spots. I place several thermometers around the room at plant level so as to get a handle on that. Humidity can be an issue, and as I have plenty of good air circulation I don't worry too much about it. Fans blowing directly on the plant can suck moisture from the leaves, could this be what is going on with your setup?
As an experiment, try spraying the affected plants with a light misting severasl times and see if the leaves start to straighten out.
I have also noticed that some bug treatments cause the leaves to try and curl up, not sure why. Have you recently applied some kind of bug spray?
Like I said, the plant looks healthy and happy except for the curl. Don't get in a panic about it. have a good look at the enviroment , humidity, temps, distance from lights, etc. and if everything looks ok, then smoke a bowl and relax.