Ideally I would like to see the part modelled as it is moulded, but if it is not possible to see the inside and you don't want to damage a sealed part then it is acceptable (IMHO) to include false internal surfaces. The part should be a complete solid and the separately moulded components should be created as separate parts. So in the Monorail Points example, the underside cover should be created as a separate part, but it does not (necessarily) need to have all its hidden detail modelled.
Chris (LDraw Parts Library Admin)