Hi,
I was lucky enough to speak to a member of the Scottish Rhododendron Society at Gardening Scotland who advised me to prune after flowering, and to do it over a three year period, i.e. a third each year - this gives new growth a chance to get going and still gives some lovely flowers each year!
I did my first prune last year, and still got a great amount of flowers - if the rain stops for a while, i'm hoping to get this years done!