Fitting dummy or sub frames to door opes is a good idea, regardless of whether subsequent door frames will be painted or not; use of a sub frame means that the door frames proper will not be damaged by other trades, not just the plasterer. Frames are particularly vulnerable to wheelbarrows! The sub frame need be nothing more than a rough sawn lining, 25mm thick and cut to the finished width of the wall (adding for the plaster thickness); ideally the door ope should be a little wider to allow for the lining and a little tolerance. Use of a sub frame speeds up fitting of the final door and frame too.