I apologies for falling prey to too easy a trap for sarcasm.
let's get back to tubes life 24/7. really? what/where/who/how could you think they need to be left on? or did you just mean you listen a lot every day?
new tubes, break in: leave them on, no signal needed, go to bed, 60 hours. off. on, warm up 10 minutes, now that is how they are gonna sound. off when not listening.
as they age, still equal l/r? have favorite music that helps your awareness of strong center imaging, equal l/r. cd version best for this, just in case cartridge is not perfectly setup. actually, have both lp and cd versions, play cd, all good, play lp, still good? great!
this is my favorite test for balance/imaging
side 2, tracks 2,3 all 3 guitars, and equal audience sounds l/rhttps://en.wikipedia.org/wiki/Friday_Night_in_San_Francisco