You mean the S curve.

Spinal S curve --> good

Spinal C curve --> bad

But maybe you're referring to the neck? In that case, you want a backwards C curve as opposed to a straight neck or forward C.

But I understand the spirit of your question. I can't answer it in a really authoritative manner, but I can tell you that the further away you are from natural spinal alignment, the more susceptible you are to a number of health problems. I believe that could include nerve compression resulting in muscular tension/spasms. If those muscles are the scalenes, now you have a potential problem with lifting the first rib and/or hard muscles bullying the softer nerves and blood vessels.

If you want more background reading, there are books from Egoscue, Joseph Weisberg, Thomas Hanna and Craig Williamsom. None of them deal directly with TOS, but they are informative nonetheless.

If you want products to aid in restoring natural curves, some people have had good results with "Back Magic" and "Real-Ease Neck and Shoulder Relaxer" products.

(Maybe you've already checked out the above, but I was too lazy to go poking around past forum posts and this is good info for others too.)

I recall one paper said that TOS people often have "downwards beaking" of C7, but didn't explain the why. I saved the quote, but not the title and it looks like I got the url wrong, so the quote is all I have:
Maybe someone else on these forums will have additional comments on the relationship between unnatural spinal curves and TOS. We'd all like to know more. Please share.