Alright, I'm using an UNO. The reset thing just has to do with the fact that I'm keeping time without an RTC. If there's a power failure setting the time again is no big deal, but I'd rather not have to set the time every single time i hook up my laptop.
There is no problem with having external power and USB power on at the same time. The boards that have an auto-voltage selector circuit will always utilize external power if it's avalible, that is it has priority over usb power. However having your arduino board reset or not is more a function of the PC software you are using and if it activates the DTR signal or not. Most arduino boards these days have the auto-reset on DTR function so if it's important that your running sketch is not reset then you might consider disabling that function and only enabling it when needing to upload a new sketch.