I wonder if I need to just drain the tank completly after every use.
That depends on several things.
What is the tank construction? ( almost all are ASME certified for pressure but the plate thickness has more to do with how much corrosion it can withstand)
Do you need instrument quality air?
Business or home use?
Coded or non coded system?
The average home use system will be just fine with even a monthly drain ( unless you have high humidity and really use it a lot)
Weekly is better, every use is even better- a timed drain is the easiest over the long run.
Lot of choices there and all of them will work and have limitations- depends really on your usage rate and how clean you need the air.
I need almost hospital grade clean dry air because of instruments and tooling but the average mechanic use can get by satisfactorily with nothing more than an "every once in a while" schedule. (and a good separator in the line)
Lots of ways to do this and the system