The original reason PG didn't wear the S-shield (which pre-Crisis *wasn't* a Kryptonian symbol) was that she wanted to stand out on her own as a superheroine, and not be confused or compared with her famous cousin, Superman.
Judging from the alternative "chest window" choice Kara made, I gather she found another, erm, "manner" to "stand out"...
She's such a "bouyant" personality.
It's hard for her not to keep her chin up.
She faces her opposition "chest forward", and "torpedoes" to victory.
Her "assets" speak volumes about her.
I guess one could say that she found a much more *ahem* "classic" symbol, then.