I dont need to police my players. My players expect to be playing the results of their choices in game, not resolving them with meta-game fixes.
My players would expect that the advantages of the t rex in combat would also mean other issues such as having to figure out how to deal with it when the story moves into towns.
Just like the wizard might prepare non-fireball spells when in a town but fireballs when more open scenery is expected.
They don't expect these issues to turn on or off scene to scene, scenery change to scenery change just cuz they made a meta-game deal.
Moreover, they would expect that a trex could be trackable, rather unique and so be attachable if the party left it alone outside town and would be rather ticked off if I told them "nah, he got to a town so that gets his t rex backgrounded, so you lost the trail."
Similarly they would not expect the backgrounding of tracking the trex defense when they got to a town.