STi Pinging?

Hi STi owners. Have any of you experienced pinging? I'm in the market for one and might be getting one this month. I've heard a detailed discussion on NASIOC about pinging with pump gas. Here's the link:
http://forums.nasioc.com/forums/showthread.php?s=&threadid=372340

Some people mix their gas but I think that's a pain. So, I'm wondering if you have heard pinging from your STi and does the ECU retard the timing? What octane and brand of gas do you use. Here in California we have to settle with 91.

Thanks for any input.

Will