Some arts have a fixed stance that is always used, be it long or short range. Traditional arts use an extended guard, modern fighting styles such as Boxing or MMA tend to use a close guard exclusively.
Here for example is a traditional Chinese Xingyi San-ti stance:
Partly because I am spending a lot of time on Xingyi forms right now, this is my preferred stance when I need to be alert to a confrontation. It has that "Back-off!" gesture and feels good at sensing an approaching threat. The lead hand is held high, the back hand is close to the core ready for protecting against kicks.
But let's face it, a good boxer can hook around the guard in close quarters and in that case we need to close our guard.
I feel that there are varying degrees of pulling the guard in as the situation changes. Here's Loren Christensen in a modified extended guard, pulling back slightly from the traditional stance and perhaps ready to rip, claw and strike.
But if someone is closing very fast, it seems logical to pull back and protect your centerline and head from heavy punches. Here's Ali in a typical boxing posture:
As Tim Cartmell instructed us at a MMA seminar, the modern method is to cover closely and shuck off the strikes with forearms, protecting centerline, head and ribs. Defense from kicks is either jamming them or stepping out of range. As Tim described, the worst thing you can do is reach out away from your centerline and core area, creating an opening for your opponent to punch through.
In summary, I think that the guard can be adjusted according to range and how much time you have as an attack closes in. By the same token, I would not unnecessarily extend a guard unless I intended to use it. No reason to signal or posture, but it is natural to raise your guard if threatened.
Now, here is my ideal of a nice training scenario:
