per '68 Plymouth FSM, 5* BTC at 650 rpm is correct spec. It should start advancing above that so its between 5 and 10* at 950 rpm. Seems like yours is working fine, so wouldn't worry about it too much.
Generally retarding the timing can make it run hotter. Later ignition results in more of the burn occuring when the piston is further down the cylinder. That heat then goes into the cylinder walls and exhaust port.
CAP equiped vehicles purposely used less advance at idle. '68 Plymouth FSM for a 225 spec's 0* @ 700 rpm.
People advance timing when hot rodding and using a cam that doesn't burn efficiently at idle. (Advancing helps because of the slower burn.) They also advance when trying to overcome the poorer performance of the later smogged engines, or because someone suggested they do it. (guilty as charged - I had several 70s /6 when younger). Finally, when running on the drag strip, engines temperatures are usually a bit cold - sometimes a bit more advance will make the make the car quicker in that situation. Factory spent considerable time and effort developing and testing the timing curves for max efficiency and power. Sometimes Dodge and Plymouth came up with slightly different specs but they are usually similar. Non-CAP curve is going to be pretty good starting point even if you want to experiment.
