Maybe I could summarize my previous post by the following rules:

* everywhere except in the list of equations of a model, evaluation
  should always be triggered, except otherwise specified (with a
  sym()-like operator)

* in the list of equations of a model, evaluation should never be
  triggered, except otherwise specified (with an eval()-like operator)

And I would add a 3rd rule:

* when trying to evaluate a symbol, if it is not assigned, then
  trigger an error. This means that it is ok to use an unassigned
  symbol in the equations list of a model, but not outside (unless the
  sym() operator is used)