I wouldn't worry about idling at 500. That sounds about right. When I had my truck before I sold it, the tachometer would sometimes drop to 0

 

As long as it idles smooth and doesn't bog or stall, I wouldn't worry about it. I started working on cars when I was 5, (I started fixing them about 10) and I've had clerks at Autozone barely old enough to shave argue with me on fixing things. I still count battery caps to check the voltage I'm working with. 3,4 or 6. I can see them pushing what they have, not what you need.

 

I just installed some Autolite XP5224 Iridium plugs and they work great plus they have a rebate of $3 a plug until the end of the year