Although I have also been charging batteries in my Pro-series Radio Shack scanners for years, without a single problem (original batteries), when I bought the HP2, I bought an external charger. After looking at what others are using, I decided on the "Ambient Weather" smart charger, for $33 Amazon Prime. It has several operating modes, including refreshing/conditioning both new and old batteries, to maximize their capacity.
But I also charge the HP2 via the usb port. I have removed the battery cover, to monitor the battery temps, and they barely get warm to the touch... certainly no more than when charhed by the smart charger at the lowest chrge rate.
I haven't heard on anyone who has damaged their HP scanner by in-scanner charging, and I don't think Uniden would design a charger that would likely cause damage, but we all know, anything is possible. So if you're worried about it, and don't mind the daily battery swaps, charge them in a charger. I'll do it from time to time, but I doubt I'll do it all the time.