It seems as if the general concensus around here is that unless I personally want my instrument rating now for my own enjoyment, there is no real benefit to obtaining it prior to entering flight training for most people.
I think I still will buy an instrument handbook and maybe even log a few sim hours with an instructor, but the majority is probably right; there are better things I could spend $5,000 on. I'm willing to do what it takes to achieve my goal of becoming a Naval Aviator, and its nice to know that an instrument rating wont immediately make the difference between success and failure at flight training.
Bingo. It's about two-three weeks of ass-pain learning the stuff, then hopefully it'll click. Don't stress not knowing the material in advance. You'll learn it, we all have. If you feel like getting ahead, go for it. Pubs are online and there are tons of gouge sites for every phase of training. But I wouldn't spend money getting ahead.
In fact, flight school is learning a brand-new topic from scratch, with little prep time, focused and compacted into a few weeks, being tested on it, graded on flying it, getting good at it, then starting something brand new all over again.
Rinse and repeat 500 times.
Edit- Let me add, while I had absolutely NO idea how much work, studying, and asspain it was going to be, I wouldn't trade the experience for the world.