Just to add to what the others posted, "nowadays" it is not as important because of the electronics involved.
One of the symptoms I would hear (on some old phones)of a reversed T&R was that the ringer armature would move with the pulses generated when using a rotary phone. So, somebody would dial and you would hear the number as "dings." The good thing about those old phones is they are very immune to Radio Frequency Interference (RFI), and they were built like tanks--drop one and you might break the floor
I would think that keeping the correct polarity is important for troubleshooting. With the proper equipment, it is possible to tell if either the T or the R conductor has a problem with it, right down to the location of and type of problem. I would imagine if at some point the conductors were mixed up, it would create some confusion.
Then again, I've never done any actual telco-equipment work so this is speculation on my part
There is a great website that has loads of information on troubleshooting: http://www.mccartyinc.com/
P. S. To tell which is which, connecting a test set from ring to ground should draw dial-tone (with a hummmmmmm....) and from tip to ground no dial-tone.