Avoid manure for rhododendrons and other acid soil plants; it will make the soil too alkaline. Further, manures tend to be high in nitrogen. This will lead to lush, soft growth prone to winter kill and will reduce the number of buds set. A very good commercial fertilizer for rhododendrons is Hollytone. Rotted bark and decayed pine needles are also excellent as both mulch and slow acting fertilizer.

If very well rotted could be used for mulch. Old enough to be used on rhododendrons would also mean old enough to not have much nutrient content. Would be more to the point to use a fertilizer, as suggested.

Don't do much fertilizing unless you have seen specific indications of a particular need, like yellowish leaf color not attributable to another cause.