Go out into a parking lot in the snow and play with it.
TCS in my '08 saved me when I had a slow leak on a slushy morning on the way to work. Other than that, it was always a tricky balance between what the computer and I thought of the local driving conditions.
Safer than sorry, don't mess with it. But if you know how to handle FWD vs AWD vs RWD, and you're willing to hit a parking lot in bad weather, then you should mess around and get used to what you like.
The 07 STi isn't as "smart" as the 08 WRX, but I have to admit that there is a bit of reassurance knowing that if you mess up, you've only yourself and your slow human reactions to blame (as opposed to questionable software written by some geek who's never driven a Subaru).