Do doctors always give patients potassium-sparing drugs if the patient has low potassium levels?

Potassium: Always is probably too strong a word, but it is advisable. If that cannot be helped, then a potassium supplement should usually be prescribed as well.

NeverSayAlwaysorNeve: If you are on a medication that tends to make you lose potassium, then the doctor could substitute or add another drug that helps you keep it. Another option would be to take potassium supplements as well as foods higher in potassium. The supplement/diet approach would also apply if for some reason your potassium was low. Other than that, it depends on your particular situation.