Hi Bob,
Well, we'll be happy to see both of you!
As for "taxing" an engine--I'm not sure what you mean. For sure, you will perhaps get a few less MPG, but how that compares to the increased wind drag of top-down and or windows-open driving is subject only to testing and speculation on our cars. On a modern vehicle, there is absolutely no doubt--highway driving with the air on consumes less fuel then window open driving. It's a mantra the automotive HVAC guys have been singing for some time, but if you look at many cars on the highway, they are not believers or the air doesn't work or they just like a lot of noise and wind blowing around!
Highway driving or even secondary road driving with your air on shouldn't cause you any issues. If you get stuck in a traffic jam, your temp might climb a bit, and you might need or want to turn off the a/c, but if you are cruising, go with comfort. Keep the wife happy!
Michael Salemi
1969 280SL
Signal Red w/Black Leather
Restored