I don't think there is definitive answer to this, and you will have to make the judgement on a project by project basis. I do a lot small works myself and once you start to develop relationships with your builders and architect it will become easier to make that judgement. 

My preference is to be appointed directly by the main client, as it keeps lines of communication and responsibility clear also you usually get paid quicker. 

However some builders and architects offer a 'complete' building regs service, so they want to appoint you directly, and there are also occasions when the client just doesn’t want the hassle of appointing another professional. 

Either way it is vital that you have a good relationship with whoever appoints you, if it is someone you have not worked with before and you’re not sure about them, you may want to request payment up front, or prior to issuing your calculations.
Me personally unless I know someone really well or it is a large project, I note in my T&C that calculations will be issued upon receipt of payment, however I will provide any pertinent design information, for tendering purposes etc. This way I give myself some security, that if I am having difficulty being paid I still have some leverage.