crockett
I think you should build for yourself and what pleases you, not "winning" a contest or a prize...if you're good enough, that will take care of itself
razorbacks: although I agree with crockett's statement in a "For-the-Love-of-the-Game" sort of way... I've been to, and submitted builds, in many competitions (from local to IPMS Nationals) and have some thoughts that I hope will shed some light on your question.
If you want to win at an IPMS events rust-and-dust don't impress. Building & basic modelling techniques DO and they are a MUST to even get on the judges short-list. Think seams, gaps, glue marks, decal silvering, road-wheel alignment, etc. etc. You can do a TOP NOTCH weathering job and get dropped if you make a "rookie" mistake. This is especially the case at Nationals where I have seen, for example, 45 models in JUST the German Light Tanks category. The judges HAVE to have a way to weed-out the models and construction seems to be the way they do that.
AMPS events are quite the opposite. Of course construction and basic modelling are important but weathering, i.e. rust & dust IS looked at closely and considered a PLUS if it's done correctly. Correctly is subjective of course....maybe logically is a better word.
So for an Abrams or Humvee: I'd look at the age of vehicle, where it served and for how long, and find some on-line pics to see what's "typical" . I expect most will be dusty (sand) but not rusty, except for some worn edges and maybe a spot here-or-there from a chip/ scratch.
Sooooo, in the spirit of what crockett said: If you're motivated to rust & dust, OR NOT, just go-for-it! Have a blast and if you win an award that's just icing on the 
That's my 
...Hope it was helpful
SMJmodeler