One word of caution. Rhododendrons don't need much fertilizer but do need acid soil. So don't apply a lot of fertilizer just to make the soil acidic. Powdered sulfur will make the soil acidic and not add a lot of fertilizer. My website below has a section on acidifying soil with sulfur under "Soil pH."

Vicki Molina in Portland, Oregon, reported after using coffee grinds from Starbucks Coffee House for about 18 months that Van Veen Nursery was very satisfied with the results. 1) it helps to aerate their clay soil. 2) slugs don't like to go through. (So you can see they have both mixed in and put on top.) It does help to make the soil more acidic. But it does not replace fertilizer.

They suspect that by making the soil more acidic you are actually helping the uptake of magnesium. This in turn helps iron uptake and that helps to make the plant green. So really you are starting a process not fertilizing. Combine the coffee with horse manure and organic mulch and watch the amount of fertilizer you use decrease dramatically. As for how they apply it, when the plant is dry and just before it rains they sprinkle it on and around. The rain takes it from there. Otherwise they incorporate into the new beds. No exact rate, just cover the top and work it in. (Courtesy of Vicki Molina)

Note: do not cultivate an established bed. Rhododendrons have very shallow roots and cultivating will damage the roots. In stead use a mulch. You should pull the mulch back when applying fertilizer, powdered sulfur, etc, then push the mulch back in place.

Here is a link that might be useful: How to grow rhododendrons and azaleas.

As with all fertilizing it depends on what the mineral content of your particular soil is before fertilizing is started. Coffee grounds will work for you if they supply something your soil could use and do not also happen to have a detrimental effect in some way.

Since coffee grounds mainly affect the acidity and have little nutrient value, there is little chance they will be detrimental unless you live in the part of the Pacific Northwest where the soil is so acidic that they have to apply lime. They seem to be one of those benign organic materials, at least for acid loving plants.

For additional observations from a .edu site...

Here is a link that might be useful: Grounds for Gardening