I think Shock is probably right, to be safe don't do anything that earns Tier Credits, at least not gambling, unless the TCs would make the difference in pushing you over a tier threshold.
Although I'll say that MLife
may work differently than CET when
using Express Comps. Hunter, the host from PH, has said that CET "expects play" on any day where TR Rewards Credits are used.
A couple of trips ago, I had a two or three night stay at
MGM Grand, and then later in the week, used Express Comps to pay for a dinner, while not staying there. It was the first stay of the year, so if they counted the dinner day in my ADT it should have affected it significantly. But I didn't see any change in my offers.
The same may be true for earning non-gaming TCs, but I don't have an example to point to.