A professional violinist tunes frequently. Even after every single piece, or every few minutes. When you play, the tension of the bow pulling the strings actually detunes the instrument. The pressure goes from one side of the bridge to the other and vice versa. When we perform we don't usually tune until after the piece but we learn to adjust on the go.
This doesn't mean that your instrument will be way out of tune, just micro adjustments for the most part. Constant fine adjustments. The only thing you need to worry about as a performer are the open strings which can't be adjusted as easily when they are out of tune. We frequently avoid open strings unless there is a good reason to use them such as chords, double stops, or that the sound is deemed musically appealing in a particular situation.