Speed wobble safe fall?
Between BMX and mountain bikes I've fallen quite a bit over the years.  If you're getting bad speed wobbles then there is something not right with your bike.  I've never seen anyone get speed wobbles on a rocky course.  If you're going fast enough on a rocky course that this is a risk you should be wearing at a bare minimum a full-face helmet.  Shoulder guards wouldn't be a bad idea either as would shin/knee combos and elbow pads.  Get a helmet, get safety gear.

The worst crashes I've seen have always been when the person doesn't see it coming, and you're probably not going to see it coming.  Popping a wheelie and landing on your back is known as looping out.  Even if you land on your feet, at speed you're going to mess yourself up anyway.  Don't do it.  I might try locking up my back brakes and skidding sideways, but that's still really sketchy and a desperate measure.

When it comes to crashing there really isn't definitive answer, and you really must use your best judgement.  You want to stay loose and judge the situation in that split second before impact.  Screw the bike, save yourself.  It's better to get road rash than to hit a semi/boulder/tree.  Be aware of the worst hazards and TRY to choose the lesser of two injuries.  Remember that if you're going fast enough that speed wobble is an issue, then losing control of your bike is one of the worst things that could happen.