Can preparing for a ground plane cause problems with TMCC signal?

If you don't hook it to anything, it's somewhat pointless.  It certainly can't be a good thing, and one thing it won't do is improve the signal in any way.

I don't think under the track is the right place for the ground plane anyway, that's where the other half of the TMCC signal lives.  The ideal spot some have said is some telephone poles with overhead wires.

The reason that there are differing opinions is due to the experience base I would imagine.  RF propagation is a funny thing, and subtle difference in the environment can have a large impact on your results.  That's the long way of saying that each location will probably be somewhat unique.

It's an odd system in that the radio signal is operating deep, deep, deep in the near field and you have a very long wavelength for the signal.   Putting a larger metal object in the vicinity of the track and not tying it ground or pin 5 will probably create more problems than it solves.  The metal could actually become a secondary/induced signal source and cause more issues.  

The house wiring provides an envelope where the signal being presented to the receiver is supposed to always be between the track component and the envelope.  You get into problems with signal propagation where the are secondary sources like over crossing or parallel tracks or secondary induced signal sources (large ungrounded metal objects) that mask access to the surrounding "envelope".  

If you have issues in "spots" than use Dale's procedure of placing a wire feed off of pin 5 on the command base (this is tied to the ground lug on the wall wart) to get the other half of the signal in the vicinity of the receiving devices.

as above; objects that conduct radio frequency change the RF field. It is hard to predict what they will do but it is a sure thing that there will be some change.

   The "ground plane" that is talked about is an attempt to improve the field and it probably does most of the time.

  Don