The following link offers good advice:

http://www.plantamnesty.org/pruning_...es_too_big.htm

Now that I've 'broken the ice' with my trees letter, I may as well
continue. You should prune out any dead wood and then stop and think.
Anything you do beyond that is not helpful from the rhododendrons
point of view. What they have now is twenty-five years worth of hard
work. They would like to hold onto that wood they have grown and
those leaves that top it. The new sprouts will grow and fill in
below. It is expertly adapting to it's situation. If you prune it
severely you may or may not hurt it much, but it certainly won't help
it. However, it may (or may not) help it's appearance.

If those bare branches really bother you each time you look at them,
such that you can no longer enjoy the shrub the way it is, then it may
then be best to cut them back and take your chances. Just think it
over first. The first rule should be 'do no harm'. Thanks for
asking.