Does the tracking wheel need to be in the center of the robot (as in the center of the wheels)?
From my experience with them yes, if you want it to work properly
If you’re using just a single wheel, it needs to be in the center of mass of your robot which is likely between your wheels, but could vary of it depending on where you put your brain and other parts of your bot
So it depends on where the mass is, not the wheels?
If you have tried, you will find that no matter the position the tracking wheel is OK to work (slightly different).
But the more important problem is the 5 wheels may not be on the same horizontal level or the floor is not flat enough. The tracking wheel may not fully contact with the floor and can’t drive the robot side to side.
Using Suspension with rubber bands on the tracking wheel will help