Should I conclude an essay on the symbolic meaning of fences by saying that fences are bad or good?
You don't say how many paragraphs your essay is, but I'm going to assume it's five paragraphs. After your introductory paragraph, your body paragraphs (2, 3, and 4) should explain the literal and symbolic fences. Be sure to give specific examples from the play and use some quotes to make your point (don't forget quotation marks). Your examples in the body paragraph should tell how the fences affected Troy and his family.
A good guideline to follow when writing a concluding paragraph is to reword and rephrase your first paragraph. The first paragraph is your introduction. Your thesis statement (1st sentence of essay) should state something general about symbolism, its purpose, etc. Then you introduce the title, author, and characters you're going to use. For the concluding paragraph, make a general statement about how symbolism is used in fences, reword your general thesis statement, give a general statement about how things ended for Troy and his family, and a general statement about how mankind is affected by fences.
Good luck! It sounds like you've got a good start already.
While sometimes it is useful to restate your thesis and summarize your key points, quite often if you have already articulated that clearly the first time, you don't need to do so again. Instead, consider the "so what" of your discussion: what have you said that is important? What have you learned about life by studying that play? what significance do fences have in our world--your world--that we / you-- live everyday? Try to avoid cliches and familiar statements at this point in your writing and instead speak something authentic and true for you, based on your view of the world, your life.
Good luck. Conclusions are notoriously difficult to write, but because it is the last word you leave with your reader, it is important to make that word original, true, and interesting.