If you are ultimately responsible then you might want an overarching document of some sort, something that compels the subcontractors to work in accordance with their RAMS just so that you are covered. I don't think it can hurt to show that you have checked competencies, training, certifications, etc for the people you are subcontracting.

Surely it will depend on the size of the subcontractor to a degree? If you are using one man band subbies they have no obligation to record their risk assessments. However, when we use larger subbies they have to submit RAMS which we then use after close inspection (when we are the PC).

When we are subcontracted for works (i.e. we hold the duty of Contractor) we submit our own RAMS for our tasks to the PC, but then we do not expect RAMS from the subbies we use at that level to carry out the works. We assess their competency in different ways and expect them to work to our RAMS.

That's how we do it anyway - hopefully that helps!!