if the crank sprocket is in the correct position, on the timing mark, the pistons are at the half way position and there is no possibility of pistons bending valves.
turning cams by hand, you would have to be very forceful to do some damage. valves bending valves is possible, i guess, but not likely, i don't think. (please correct me if i'm wrong.) and since the nature of the cams is to "close" the valves, the tendency is to "avoid" damage.
having said this, is it possible that you could force the cams around and cause a problem, probably, but if you set it up as it should be before you remove the belt and do not mess around with the cams too much while hanging the new one you should be ok. lots of first timers have done this successfully. just make sure you are using the correct timing marks and you should be ok.
there are links below in my signature to some great pictures and articles for a timing belt change. read up and have fun. if you have more questions , ask.