This depends on so many things, if you're power walking then you should do some stretches before hand, if it's just walking then you can just get up & go, of course this is if it's first thing in the morning, later in the day you've probably warmed your muscles up enough to power walk without warming up.

As to whether you should do them afterwards or both, this depends on why you're exercising, when, how long, & whether you're getting enough nutrition!!