What do pianists do when they need to perform after many performers who sweat all over the keys and made it slippery. I was thinking of using a personal cloth to wipe the keys, but would that be peculiar and unmannerly? BTW, I am having a piano performance soon.
Speak with the stage manager beforehand and ask him or her to have a stagehand check the condition of the keys after each person's performance and clean them off if necessary -- or, better yet, clean them off after EVERY person's performance so as not to make it look like one particular player was at fault.
You, the performer, should not be cleaning the keys on stage, just as you should not be the one to push the piano out or to sweep the floor.
The exception is when the event is so low-budget or self-organized that the performers DO sweep the floor and move the piano onto the stage; everyone is his own stagehand.
It depends on the venue and audience to determine proper etiquette. Is there an open or closed curtain? If there is a stage manager that would also be appropriate. If not I wouldn't see any problem with using a personal cloth. It's important that you feel comfortable and confident in your performance.
If you need to adjust other things like the bench or lid, it would be appropriate to make sure the piano is in a proper state for you.
Wipe it, or ask someone else to wipe it. Etiquette doesn't matter, your performance does.
I remember an occasion when I was MD of a show, playing onstage. One night my band thought it would be funny to cover the piano keyboard with shaving foam. I walked on, took my applause, looked at the piano, walked straight off again indicating "Hold on a moment!" to the audience. I really wish I could have thought of a good gag, but failing that I just sent the stage manager on to clean up.