CO-meter indicates 10ppm of Carbon Monoxide when I turn on Cabin heat - should I be concerned?
The short answer is you should be concerned.  

Although 10PPM of carbon monoxide doesn't pose an immediate health risk for short term exposure the fact that the level is rising when you turn on the cabin heat &amp; falling when you ventilate the cabin (by opening the door) indicates you may have an exhaust leak, and those tend to get worse over time.
You should have your shop open the heat muff &amp; inspect the exhaust system for cracks/leaks before your next flight (or if that isn't possible avoid (or limit) use of cabin heat &amp; defrost &amp; monitor the CO meter as part of your scan until the exhaust system can be checked).

You should also check your CO meter: electronic CO meters have a finite life, just like chemical spot detectors (though they generally last several years) - your detector's manual will specify a replacement interval. As the sensors age they may give inaccurate readings.



A note about ground operations - Electronic CO detectors are sensitive enough to occasionally generate transient alarms during ground operations due to exhaust gasses entering the cabin. This can happen during summer or winter.
As long as the levels are low and the alarm clears when you're moving (taxi or during climb-out) this is probably not a problem, but you should know what is "normal" for your aircraft.