Whoever at HTC that told you that is an *****. Typically, manufactures (phone or otherwise) will only design power supplies/chargers Just strong enough to run the device. So if the factory charger is rated 1.5A, then you don't want to go anything less. The only exception maybe being charging the phone while powered off since it's not needing to power the circuitry. It's fine to go with a stronger charger because the phone is what determines the power draw. Compare it to the wall charger plugged into the wall. The house circuit is rated at least 15A, but you know it's not forcing that to the charger and phone. It'd blow up!
A stronger charger may help charge a little bit faster, but I've never seen a noticeable difference because the phone is setup to draw a specific amount. I do prefer strong chargers, though, for flexibility and so it's not stressed as much as one that just meets the minimum. It'll run cooler and tend to last longer. On the flip side, once that is too weak and rated below the stock charger could overheat and burn out prematurely because the phone is demanding too much from it.
I've also seen manufactures claim you can only use OEM chargers because the phone needs the right type of signal or something like that. Motorola claimed aftermarket ones wouldn't work at all with my Bionic, but about all I ever use is an aftermarket car charger with no problems. There may be some durability or electrical noise qualities that make an OEM charger "better" in a pure technical view, but not enough to be worth the premium they charge and probably nothing the end user would notice. That's my opinion, anyway.