hi chris, just missed you in the chat channel.
in general its safe to put anything "above normal" priority, and you should not put anything as real-time priority.
having said that, most of the time what you want to do is leave everything normal and not adjusted -- and just look for processes that use a lot of cpu (say over 20%) and set those to low.
process tamer will automatically lower the priority of processes that use too much cpu, so you dont really have to do anything manually. but you could pay attention to what process tamer does and force something low if PT is changing it automatically often.