If you're like me, anything I sing out loud is a mistake as I have a terrible voice!
I think the worst songs are ones that require a big vocal range - and for some reason, these ones are always the most tempting! Whitney Houston "I will always love you" or Mariah Carey "Without You" are those big power ballads that everyone likes to sing in the shower, or karaoke, but probably shouldn't!
My advice? Keep it to the shower and you're fine! Although there's nothing wrong with having a rubbish singing voice- even oscar winning actresses like Jennifer Lawrence admit that their talents like elsewhere (although something tells me that if she turned up at the American Idol auditions, Simon Cowell wouldn't exactly send her packing....)