In my experience they rarely need replacing unless they fail.I would keep a spare on hand though because when they go bad they usually just stop working with very little drama (although the other tubes are not going to light up either).
If it makes you feel better you can remove it from the socket whenever you change the output tubes and shake it next to your ear to make sure that none of the internals have worked loose. I had one rectifier tube that was sketchy and had a piece loose inside- sometimes the amp would work but after moving or transporting it it wouldn't work at all...until I moved it again.
If it makes you feel better you can remove it from the socket whenever you change the output tubes and shake it next to your ear to make sure that none of the internals have worked loose. I had one rectifier tube that was sketchy and had a piece loose inside- sometimes the amp would work but after moving or transporting it it wouldn't work at all...until I moved it again.