My standard policy, informal as it may be, is to at least check out the problem and be as helpful as I can be, within reason.
If the error is my fault (on the very rare occasion), sure, I'll take care of it without comment or question, then appologize profusely.
If it's a product problem, that I recommended or supplied on my decision, I will replace it within a year or so.
If it's some cheap product that the client insisted on, then I will have to charge them for everything, although I may cut them a slight deal on labor to keep them happy.
In this economy, you are forced to be a lot more polite and "reasonable", even beyond what you would normally find "reasonable".
I find my self having to decide if it's more advantagious to lose a bit on iffy warranty work, or risk losing a customer or their possible refferals.