It is not required to accompany your proposal with results from simulated ALMA observations (see Section "Science Case"of the Proposer's Guide). However, there are several situations that it may be wise to use CASA's simulation capabilities to understand the properties of ALMA observations.
As described in Section "Response and Snapshot" of the Technical Handbook, CASA simulations can be useful to disentangle sidelobe effects from real point sources, especially with relatively strong point sources. In the following section, it is pointed out that estimating the distribution of power at different length scales for targets you wish to observe through simulations may be helpful.
If simulations are discussed in your proposal to justify the technical aspects of the observations, then images and simulation details should be included in the science case.