Replacing 4 instead of 5 tires on a used trailer?

When I replaced mine, tire dealer said my spare (covered and never put on ground) was in better shape than others and not to replace, just be aware if I used it, to take it easy on it remembering it was older than the others.

I figure if he wasn't pushing me to buy a fifth tire, then it really was okay. This was at a locally owned tire shop. If they don't sell tires, they don't stay in business.