I'm working on a web app project (in java; not that it matters) and we have a form with drop down lists and input fields. 

Obviously drop down lists are provided because we expect a specific value from a set of values. 

So my question is this: does it make sense to ensure the submitted value is in the set of expected values? Or is it acceptable to just assume the correct value is coming across?

There aren't any "errors" that would arise from different values being submitted,
 but the data store would not be consistent with the business rules/requirements.