I haven't always had my current policy but here it is:

Always respond politely and constructively to every trade offer. You always want other owners to think well of you when they're looking for trades, and there's LOTS of room for interpretation when evaluating trade offers. Context matters, and if you get into conversation with somebody maybe they realize they didn't offer value for you. Some owners have a modus operandi of offering unbalanced deals and working down from there...hey, if it results in a good trade for me why not talk about it?

I'm trying to come from the point of view that more conversation leads not only to more trades, but a better community, so why not address every offer with an intelligent response?