Marilyn, I looked back over our notes to each other and in them we decided that 'slightly over or under' is acceptable. So I'd say a few words over or under would be OK, but to me 10 is a few. So I'd try to stay as close to 150 or 500 as I could, mostly out of respect for those who have already submitted theirs and kept within the suggested word frame.
I hope this answers your question. Juli or Char might have a better answer, but I'd say let's stay right around 10 because that could amount to a complete sentence that you might need.
