The process is called degassing. This is done to prevent arcing in high voltage transmitting (power amplifier) tubes. It is not as important for lower voltage tubes but I am sure if done correctly no harm should result.
To degas you preheat the tube when it has been inactive or in storage for a long period of time. Residue of gas is removed or absorbed by getters (elements such as zirconium placed in the tube to absorb gasses). As a result of degassing, the cathode regains its optimal emission. If that is omitted, there is a greater risk of arcing (flashover) between anode and other internal connections. Preheating can be done using the tube's heater filaments for a period of time prior to applying high voltage to the anode.
There are a number of resources on the Internet that describe the methods and why it needs to be done. I myself have actually never done it. See W8JI's web site: www.w8ji.com.