A local rebuilder of cooling units told me that it was the people that turn their fridges on and off between trips of every week or two that have the most problems with cooling units cracking and leaking (they quiz everyone that brings one in). They went on to tell me they believe it's the hot/cold cycling of the tubing that stresses it and by leaving it on it minimizes the fatigue.
FWIW, they also told me that they have done tests and found that running a fridge out of level starts to cause irreversible damage after as little as 15 minutes.