Are they showing some symptoms that make you think they are lacking something? Fertilize them only if they need it, I've never fertilized mine but top dress them with compost each Spring. Rhododendrons are not particularly heavy feeders, frequently need no supplementing with the natural acidic soils most of us have here in the PNW.

If your rhododendron look healthy and produce flowers, they don't need it, and fertilizing wastes your gardening budget as well as possibly producing an abundance of soft foliage that will attract pests. If they do show signs of lacking fertilizer - Apply an organic rhododendron food just as the plant begins to flower - this could be April or May for some, rhododendrons will be in active growth just as flowering finishes . Hollytone (Espoma) is one thats recommended and while I didn't see it in nurseries for years, I do notice it on the shelves now.