the instrumental shouldn't be mixed till the vocals are added..
you should always have rough mix of your instrumentals so artists can feel the vibe of the beat and hear the different sounds...no you can put the beat in song structure of let everything play through and then once the artist perform over it break it down add scratches,vocal bits whatever-post-production
now that's your a producer you do the above
if you are just a beatmaker looking to sell beats then of course you get the instrumental mix...for leasing instrumentals
any serious artist will want a exclusive instrumental with trackouts