o2's
I am guessing you are talking about narrow band sensors. If you are running a single I would leave them pre turbo, accuracy will go to crap in boost but in cruise (when they are needed) it will be fine. If you are running twins, you can put them pre turbo or in the downpipe, but plan on them being a little off in the downpipe as it will kind of be like long tubes where the readings can be a bit off.
If you are running in full time open loop mode that would be true if you don't want to give the computer any ability to trim itself if you are running lean or rich during cruise and idle or if the wideband is being used in place of the narrow bands through a narrow band simulator.
I ask because I dont have o2 bungs in my merge pipe. Plus im cleaning up the engine harness and wasnt sure where to place them. I was told the added back pressure and heat make o2's fail and could go into the turbine.
Subaru runs one in the pipe about 8-10" right before the turbo from the factory. I have seen Subarus with over 200k miles that went through 3 o2 sensors without problems. Works fine and I have never seen one come apart and go through the turbo. Maybe just make sure you have a good OEM quality one 
