My thought is all tanks with fish need a top! Clowns are not known for jumping and I have had 3 jump (lost 2 of them) FISH jump and I feel it is our responsibility to prevent it.
Just to follow up I went with three juvenile Chromis and I couldn't be happier. They've been in there about a month and already have made the other fish more comfortable and exploratory. The two clowns often come out and swim around with the Chromis. They're goofballs though.