In addition to the previous post: The best way to deal with datum planes in an assembly is to have a default datum plane layer in your part and assembly template files. This will put all datum planes in your parts and assemblies on this layer automaticaly. Then, in the main assembly, open the 'Layers' dialog box and it will give you individual access to this layer in each part and subassembly. You can selectively blank the unneeded ones and then save the layer settings.
-- Alex Shishkin