If it is in veg, rain won't hurt it, even if its is in early flower it won't hurt. However if it is in late flower, yes cover it, but make sure some sides are open and to let air in, if it is totally enclosed, the plant will just sweat and cause as many problems as if it were out in the rain.
At this stage you have basically created a "sweat lodge".
Cannabis has survived for Millions of years without tents.
Hope this helps, Organoman.