That's for you to decide. I only warm up (this applies for piano also) when I feel I need to, like when it's cold or I'm tired, but otherwise you can just start playing what you would normally and you'll pretty quickly reach your peak.
Some people go the way of picking exercises and stretches, but I really don't see the point unless it's before a gig or something.
Some guys (John Myung [Dream Theater]) even warm DOWN after a show, so basically, there is no definate warmup 'method', just do whatever works for you.