You are correct: You do not have to leave the AC settings on all the time. It is a good idea to of course run it at least once a week or so to keep the compressor seals lubed etc.
If what he said were true, you would not be able to enjoy a setting that excludes the AC!
On our '87 I found the temp sensor most forward, on top of the engine was failing. In the process of replacement I noticed a temperature rating on both the failed original and the replacement. The original had I believe a 104 C rating and the replacement (which I don't believe is a correct one) has 107 C rating.
You may want to check this sensor.
.... and hey, enjoy your sunroof, the breeze blowing through the windows!
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)