Hi,
The nocheck option is generally used in models where you have random walks with drift, ie equations of the form:
a_t = \alpha+ a_{t-1}+\varepsilon_t
There is no steady state for a, except if \alpha=0 in which case there is an infinity of deterministic steady states. In this situation (where the steady state does not exist) you can provide an arbitrary value for a in the block defining the steadystate or in a steady state file routine. This is safe as long as the elasticities and the ratios appearing in the linearized form of the model do not depend on a. Since there is no steady state, any value you provide for a is wrong, hence the nocheck option.
Best,
Stéphane.