You don't, but you might play them sequentially. It's just something to be aware of, whether the strings were tuned to equal temperament or just intonation: if

You don't, but you might play them sequentially. It's just something to be aware of, whether the strings were tuned to equal temperament or just intonation: if you're in the key of G, that open E might be a little off from the note you want. In practice, I think experienced violinists tend to avoid the open strings. This may be one of the reasons why.