Remember, rhododendrons set next year's flower buds on this year's new growth. When applying fertilizer, gently scratch it into the mulch beginning six inches from the trunk, working your way out to the drip line. If you are going to apply more mulch, fertilize first then mulch on top. A good rhododendron should have multiple branches close to ground level, froming a well rounded plant. Heavier pruning can be done when necessary.
Many problems result from planting a rhododendron in an unsuitable spot.
So plant the low growing ones where suitable, and the large ones where they have room to grow. Fortunately, rhododendrons are shallow rooted and are relatively easy to move. If you are not sure which rhododendron best fits your location, just ask.