I would at least cover the inlet piping with a sponge to keep from sucking up fish or having a snail try to become a cork. I had a snail make it in my overflow once and very nearly had a large overflow as the sump kept pumping but there was very little water making it back...very annoying. But I don't think that an overflow is required. However, if you don't have an overflow somewhere (like a HOB skimmer) then you are more likely to have scum float on top of the tank. You can minimize with active pumping toward the surface, but an overflow works much better.
