What is the typical CPU utilization of idle iMac?
A new Mac may use a considerable amount of resources building the database for Spotlight. This may take several hours. but on a new machine, without a lot on it, maybe as low as 30 minutes.  If you see a lot of resources being used I'd be concerned. My older iMac is currently just sitting here doing nothing but I have 7 web pages open. Activity monitor says the System is being used 6% of the time, user uses about 20% of the time and its ldle the remaining 74% of the time. Is this consistent with what you are seeing?  These values are given on the CPU page to the right of the CPU Load plot.

Even when you are doing nothing, many processes are running in the background as you can tell by the amount of processes listed when you use activity monitor.