I always thought reusing old pad was fine, but a family's allergy issues made me rethink
For quick jobs, I used to lay new carpet over old padding if it looked decent. Then a customer's kid started having bad breathing problems, and their doctor traced it to mold in the old pad. Now I never reuse padding, even if the customer asks to save money. I explain it's a health risk I won't take on.
Heard about a friend who redid their floors last year. They kept the old padding to save cash, and later found black mold growing underneath. Their whole house smelled musty and the kid got really sick.