In my view, the right answer is definitely to never friend.
If you never friend, they’ll assume either that you hardly ever use Facebook and you didn’t notice their request, or that you have a policy of only accepting friend requests from people you’d consider real friends (as opposed to just acquaintances) — and there’s nothing wrong with such a policy. Everyone should understands it’s kind of ridiculous to friend every person you meet, even though that’s what most people seem to do.
If you accept the friend request, they will receive a triumphant notification indicating that you’ve accepted the request. If at some point they decide to look at your page and see that you’re no longer friends, they’ll think you looked at their profile, saw something that made you dislike them, and then de-friended them for that reason. In other words, some people will notice being defriended, and interpret it negatively in an insecure or paranoid way. Never friending is less likely to hurt someone’s feelings.