The correct fee is so small right now that the psychological detriment far outweighs the benefits of charging it imo. It’s like that damn place that charges me a quarter a tiny cup of water when I buy a $9 meal. Yes, I know there are costs to pressing the button, but now I don’t like you anyway.
I expect it will be a long time before a high % of nodes require a fee for normal transactions. This means most users will not attach a fee, being willing to wait 4 blocks on average or whatever. So for a good while charging a fee simply means not putting transactions in blocks, this will make bitcoin slightly less appealing and slightly reduce use which will slightly reduce the value of coins. So anyone who has coins can self interestedly include transactions for free.
Jgarzik, do you have a guess as to the resource costs of a normal transaction?