Always use the seatpost which is designed to be clamped, please do not damage your bikes with bad advice. Do not think "oh I will just do this one thing and be really careful" because you easily damage something and not notice. Certainly yes some older bikes and heavier duty bikes might have more robust tubes but they still were not designed to be clamped. Seatposts are actually designed to be clamped it is how the stay in your frame is through clamping.

In terms of bags getting in the way, remove those bags, Topeak makes the Fixer which just clamps on to your saddle rails and allows you to slide your seatbag on and off very quickly and keep it quite secure. Otherwise generally they are simply hook and loop fasteners and don't require a ton of work to remove. One should not intentionally damage a bike simply because removing something minor was just too hard or we are just too lazy (really the case). Many good rear lights these days are silicone or rubber fasteners and you still will see some one attached with a screw and plastic clamp but those can be move out of the way pretty easily with a turn of the screw or you can change the height of your seat post briefly to work on your bike great thing about that is you can find out if you have done your job of greasing it properly so it doesn't seize and might just be a good opportunity to clean it out and re-grease. you can also use a marker or a blade and just mark where your post was or measure with a tape measure and write it down in case you need it.