That was one of the things that got this discussion going a lot stronger.I charge a diagnostic fee on all warranty calls regardless of brand and whether or not we installed the equipment and it is in my proposal.
As for your last question, my answer is still no I do not charge a trip charge or diagnostic or anything if it is still under the parts & labor warranty. The only time they would incur a charge is if there was some sort of abuse that caused a problem or if someone else worked on the unit & did something that caused a problem. I want my customers to call me because they're comfortable with my service & trust me. I never want someone to call me because they feel obligated to.