Well, aye, it's kinda surprising actually. It's like, the one and only question that's being asked everytime a new member joins
Anyhow, even though Diegio has a point, I thought I'd atleast just tell you since that hopefully might give you, and even more people the answer to the question.
While working with a highpoly/lowpoly model, I commonly start out by doing the highpoly model, and then model a lowpoly which I know will fit in well with it.. but this is no rule or better way to do it really. Some preferes doing the lowpoly first.. It pretty much comes down to what suits you the best, and what kind of workflow you'll have.
For detailing something in ZBrush for example. I usually find creating a lowpoly model in max, importing it to ZBrush, sculpt it, export lowest subdv, unwrap, import again, and render out the normals to work the best
So in conclusion, simply do as you like.. there's no "Better or Proper Way"
Cheers!