It's common knowledge that good records should be maintained and receipts should be obtained related to routine maintenance during the warranty period whether you do your own work or have others do it. That way, when a warranty claim is filed you will be able to prove that the maintenance was performed.
I take it a step further and keep the used parts. In the event of a warranty claim, the manufacturer can look at my maintenance log, all receipts, photos of the work being done (and odometer reading) and the actual used part. So far, I'm stocking away the oil filters and the crush gaskets for the drain plug that are required to be replaced with every service, the first set of spark plugs and the air filter.
You're wondering why I don't save the drained oil too? Well, heck, I'm not anal. :)