From RDH4:
"(1) Unless otherwise stated, it is desirable that valves should be mounted base down and in a vertical position. When it is necessary to depart from vertical mounting, the plane of the filament of directly heated valves should be vertical. Simiraly, the plane of the grid side rods (or major axis of the control gris) of indirectly heated valves having a high transconductance and/or a long unsupported supported cathode, should be vertical. This reduces the possibility of filament - and cathode-to-grid short circuits and microphony in filament valves."
In real life, do we care?
No.
Never had any problems. Maybe a guitar amp that vibrates a lot might cause problems. Tubes are cheap. In a 1 RU rack you have to mount the tubes horizontally. If your like Kevin C who confessed to me at the AES show that he owns 2500 vacuum tubes, do you think he worries? Bastard! :razz: