Sealing painted walls before papering?

Hello,

I am re-wallpapering my house, and have already taken the old wallpaper off. Prior to being papered, it is clear that the walls were previously painted with emulsion.

In a few places, where I took the old paper off the paint came away with it, leaving bare plaster. In most places however, the paper came off much more easily leaving the paint intact.

Where the paint came off, I am intending on sealing with a PVA:water mix (1:5) before wallpapering - my question is, do I need to bother doing that on the bits of wall (the majority) where the painted surface is still sound?

Thanks,

Ackoman