It’s best if you address the high frequencies of vocals and other instruments at the mix stage if they sound harsh or clash with other notes/chords/timbres.
A mastering engineer has specialized sibilance and resonance reduction tools to deal with overly harsh mixes. The trouble is that in the mastering stage, all the instruments are blended together and we have less control over the sibilance of single instruments. If we attempt to reduce sibilance from a particular offender, there can be unavoidable touches to the other instruments in the same frequency range.
If you don’t feel comfortable doing it yourself, consider talking with your mastering engineer about stem mastering. This will allow your mix to hold its integrity while giving the mastering engineer far more options to tame these wild horses.
I specialize in stem mastering!
Please get in touch if you want help.