If you really plan on keeping it. I'd say not a waste, especially where you live!
If you go under there and look now, you will see seems where body panels meet. Salt and dirt and moisture gets into the seems and it will rust corrode and all that mess. Mind you, 6 years you will never notice but long term you will.
Rustproofing will fill and seal the seams, making it easier to clean up after a wheel because dirt never got into the seams. It also makes it easier to work on take apart wise in the future and deadens sound. Plus, does add to the resale. And it looks good!
That's not a bad price.
Do it before winter but Mods will make for touch ups! See what the cost of that is!
MC