I rarely use it, but when I do, it’s Beverage - 1 (for tea), Defrost (I forget the different options) and Frozen Entree - 10 oz. (I just used it to reheat frozen lasagna.)
Our microwave has two settings: on and off. There are no buttons except for the one to open the door, only a knob that you turn to the desired time. To turn it off you turn the knob back to zero.
Always full power, I use the 30 sec. quick launch button a lot.
Side note: I don’t remember where, but I read an article online about laziness and one guy wrote, “When I need to use the microwave for 1 minute I always push 1:11 because I don’t feel like reaching all the way down to push the zero button.”
Thanks to this thread, I have been experimenting with different power levels on the microwave. Turns out it’s actually possible to make porridge in the microwave without it boiling over if you use a lower power setting for longer. Thanks to the SDMB, my mornings are much easier now.
I use 50% power almost all the time. Think about your regular oven - 350 degrees is the most commonly used temp and that is considered to be medium heat. You wouldn’t reheat leftovers at 500 degrees in your oven, so don’t reheat them at full power in your microwave either.
I only use full power if I have a frozen something and the instructions on the package say to do so, OR if I’m cooking something from a raw state and not simply reheating.
Guess what? Leftovers still taste good when they are reheated properly!
I’ve never used defrost.
I use the power buttons when I reheat food or it would be hard in some spots and cold in others.
Otherwise it’s full power (steamed vegetable bags, popcorn, boiling water).
This reminds me of back when I was in high school, and all my friends’ mothers were signing up for weird “microwave cookery” classes. They were new tech, and my family didn’t have one.
Anyway, I ended up getting one as an adult in the 90s. Cooking? Ha! Couldn’t do jack with the things. I’ve only ever used them for simply nuking the crap out of stuff. Well, that’s not entirely true - I’ve tried defrosting a few times, but it never seems to work. I got burnt bits and still-frozen bits. Nuking to heat stuff? They are brilliant. Anything else? I’ll use the stove top.
I have a 1250-watt microwave, so when I’m cooking something with directions written for the standard 1100-watt models, I’ll drop to 80% power. I’ve also used lower settings for tasks like softening butter. I don’t normally use defrost because I dislike the results.
My microwave has a “sensor reheat” option that figures out time and temperature needed to heat my food for me. Nice when I’m not yet completely awake. 
I mostly use the “add 30 seconds” button. But occasionally I use the power level button (60 or 70% usually) to reheat largish items like meatloafs or roasts.
I’ve used most of the different power levels at one time or another. I don’t think I have any rules of thumb - I just choose what I think will do the job depending upon what I want to cook or warm up. Very inexact, perhaps, but no one has died from my cooking yet…
I think, but I’ve never tried it, that I can target and launch America’s nuclear ballistic missiles with all the buttons on my microwave. That’s why I’ve never pushed “start”.