Whenever the opportunity presents itself, you can take the stairs. It will not kill you - only make you fitter, healthier and stronger.
I personally opt to take the stairs. My kids love to push all the buttons in the elevator or fight about who will get to push the button - so to avoid this we take the stairs. Okay if we have to go up 30 flights I would probably take the elevator - or if I have a stroller.
If you work/live on a high floor perhaps you could start by taking the stairs on the way down and maybe once a week take them up and work your way to taking the stairs 80% of the time. You could then cancel your gym membership. Saving you money.
Using an elevator requires energy (electricity). But then you say - It is using the energy whether I am on it or not. But if you are the only one on it - then the energy is just for you. And if you take the stairs it may inspire others to do the same.