Absolutely.
And while a determined 'cow' will find a way to stick its head through the wire and eventually re-stretch it....it is less likely they will do so if the wire is tight. Also, using fence 'stays' can help keep fencing in a condition that is less inviting for livestock to stick their heads through.
It is well worth the effort to keep barbwire tight but doing a mechanical re-stretch of the entire length just isn't feasible many times. Hence the need/practice of 'crimp' stretching....which works well enough in most situations.