For a steel, if its at or below working pressure, leave it alone.
If its an AL tank, blow it down to 100-200 psi. Why so low? Because in a fire AL tank alloys will change properties at about 350F, and the result is about 1/8th as strong as the original! The result will be an instant explosion.
If the tank is full, the burst disk will PROBABLY blow first. However, that's bad for AL tanks, as they are succeptable to sustained-load cracking. If the tank is PARTIALLY full, however (e.g. 2000 psi), the disk will NOT blow off in time, and the tank is extremely likely to explode first.
This does not apply to steels.