It's important to know the range of the music.

It's important to know the range of your voice.

If your voice range doesn't fit the key of the music your are playing, then you need to transpose the music into a key that is suitable. Once you've got the key of the music, there is nothing wrong with playing a note or two, and humming a note, to help you enter the song. Sometimes when I'm playing many songs in a row in different keys, I have to stop for a minute, and play a note, just so I'm sure I'm entering it right. It's more difficult when you are singing and playing music with people with different voices. As a group you must decide what the key is for the song. I always have problems when trying to play a lot of Beatle songs with people that have Soprano voices.