If you wanted the absolute most appropriate reading you would want to read what is going directly into the turbine. However, since its damn impossible to get any type of probe in there, you move down the exhaust stream, and whats at the end of the up pipes? the manifold.
Every taps the pass side because its easier to do. I have aftermarket manifolds and tried switching the probe around, no noticable difference on my gauge between driver or pas side
I am sure that putting a probe in the exhaust will not cause a restriction in the flow. The manifolds are made to take advantage of pulse tecnoligy, that was figured out by old timers.
That is why headers do not work on a diesel. Put the probe post turbo and you will still get Knowledge, if you need to have big numbers, bend the needle.
Ive thought about adding one to the drivers side manifold also. To see if there is ever a strong difference between the two. Like if there was an injector on one side doing something it shouldnt. It's mostly an idea that developed in my head when I started to think that my knock is on the drivers side, but my probe is passenger side. Numbers are fine passenger side, could be a different story drivers side, hopefully not.