Wideband O2 Placement

What is the recommended location to place a Wideband (AEM or Innovative) if there isn't a bung available? I'm specifically asking about WRX's. I do know there are aftermarket downpipes available that have bungs directly behind the turbo. There is an 02 Bung behind the cat. Is it acceptable to place it here? What about in the exit of the tailpipe?

Sorry for the newb question but I've been looking around and haven't found much.

TDLR: Anywhere before the cat.

From what I've read you have two concerns with your O2 sensor placement

1.) Accurate Readings

2.) The lifespan of the sensor

For 1.) the wideband must be placed in the downpipe BEFORE the catalytic converter on the downpipe to get accurate AFR readings. If the catalytic converter is at the back or end of the downpipe and you have some choice on positioning the wideband. There is some debate online about what position is best (closer to turbo or further away) but it shouldn't really matter assuming the piping going from your headers, up-pipe, turbo, and into the downpipe is a closed system (which it should be). You might also want to consider where to put it based on ease of access/install. etc.

For 2.) The lifespan of the sensor may be shorter if its subjected to super high temperatures and heat cycling (eg: right next to the turbo).

Let's don't forget that the closer O2 sensor located to the head, the more sensitive it will be to changing ignition timing (up to 0.5 AFR difference) whilst the actual air fuel mixture will stay the same...