I agree totally re your point of not understanding of the domain, it can be really hard to make good conceptual models until you've actually tried throwing some code at the walls.

I guess I'd rephrase your point about "one to treat as a throwaway" as saying that you should realistically expect to be moving back and forth between architecting the conceptual model and implementing it. I find it useful to first think about the conceptual model, then write some code for a while, then revisit the model and see what difficulties I've hit and/or what new good ideas I've thought of in the process, etc.