I think I mentioned before, what is in the lead, be it lead voice or instrument I run center. I slightly pan harmony vocals on a final mix.
my normal procedure:
mix vocals and check by turning down volume gradually. should be able to hear all or none, not one or two.
slightly pan harmonies to left and right and listen in headphones. sound should be balanced left to right. adjust volumes on harmony vocals until evenly balanced left to right.
by panning, usually the panned track will increase or decrease in perceived or real volume (mine have normally decreased in the total mix. The amount of volume/presence difference is usually about the amount needed to keep the lead voice only lightly in front of the harmonies.
go back to speakers (preferable flat response monitors) and turn volume gradually down again checking for lead vocal should be last to be heard as turning down.
this is also how I mix an entire song. by turning down the volume you learn what really stands out in the recording. One of the biggest things is flat response speakers or headphones. Use of something that “colors” the sound only makes it harder to mix cause it throws everything off.
example: turn “rider” down (wearing headphones) until you can only hear one thing/track. What do you hear? without trying this I’ll guess bass, because it is overwhelmingly loud on all my playing apparatus except the cheap computer speakers.
Now turn the volume to where you can barely hear the loudest lead (vocal or instrument). Can you hear all the others at that volume as clearly? you should be able to.
finally I use this trick with volume level to create the different layers of sound that make up the intire recording. I decide what I want in front, middle and back, then use this changing in my listening volume to double check my guessing I’ve done at more normal volumes.
hope some of this makes sense.