Strictly speaking, you can do whatever you want & especially, whatever works for that particular song in the style you want to do it - which doesn't specifically answer your question, I know... :-)

Me, I'm having enough trouble just pushing the buttons and things to worry about singing at the same time yet...

A simple drone on the chord buttons only can be useful, but you might want to phrase it in keeping with the song - i.e. don't just let it drone on permananently... and/or use the chord buttons on the rhythm accompaniment with or without using the tune on the keyboard - or you can use the keyboard for the chordal stuff instead of the chord buttons - you will get 'piano style' chords then (with whatever inversions you want) instead of the fixed format chord you get with the buttons - also you can get totally different chords if you hand finger them - like guitar chords...

Robin