The cause for moving rhododendrons to shadier areas is after they get attacked by lace bug insects or show signs of sun scald. Some varieties are susceptible to lace bug and some are susceptible to sun scald.

Whitish specks on the upper surface of leaves and dark spots varnish-like on the bottom are symptoms of Rhododendron Lace Bugs, Stephanitis rhododendri, and Azalea Lace Bugs, Stephanitis pyrioides, small insects with transparent wings on under-surface of leaves.

Large leaved rhododendrons are susceptible to sunscald if the plants do not receive enough moisture before the soil freezes. The leaves curl in cold winter weather, exposing the central part of the leaf but not the edges, resulting in the area near the main vein becomes desiccated and turns brown. To prevent this, water plants thoroughly before the first hard frost, protect from drying winds, apply mulch, and locate in partial shade.

Unfortunately there are several different clones in the trade labeled Scintillation. The one I have is definitely susceptible to sun scald. It doesn't harm the plant, but is unsightly.
If yours look OK then there is no reason to move them.

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

Thank you, Rhodyman. I am not seeing any signs of lace bugs or sun scald. But the Holden's flowers look a little crisp/dry on the edges and are not lasting very long, so I'm thinking that might benefit from more shade. I will also move the Scintillation rather than risk scalding. I have only planted all of these within the last 1-2 months, so they have not been through a winter yet.

I think I will try to keep the Nova Zembla there. Which other hybrids are known for sun-tolerance? (I've read that Vulcan Flame is).

Thank you.

You cannot grow Rhodies in Atlanta in full sun. Your best bet for survival and flowering is a spot with high shade and two of the best varieties for the south are Catawba and English roseum.

I agree you may have difficulty growing Catawba and English Roseum in full sun in Atlanta, but you can grow Phytophthora Root Rot resistant varieties like Peppermint Twist in full sun. I just came from the American Rhododendron Society Convention in Tulsa, Oklahoma. We had people from Oklahoma and Texas that have found varieties that will grow in full sun. They are Phytophthora Root Rot resistant varieties. Most have some R. hyperythrum in their lineage. Another important factor is a mulch that will help keep the roots cool and excellent drainage. The main killer is hot root zones and Phytophthora. They also use a lot of bark dust in their mix and very little peat. This helps prevent root rot also.

I just read an article by Steve Bender in Southern Living April 2008 edition that recommended Anah Kruschke, Anna Rose Whitney, Ben Mosely, Blue Ensign, Calsap, Caroline, Cynthia, English Roseium, Janet Blair, Scintillation, and Wheatley as being heat tolerant. Also an article at Clemson University that recommended some of the same ones plus Album, Ginny Gee, Lee's Dark Purple, Nova Zembla, P.J.M Hybrids, Roseum Elegans, and Vulcan's Flame. It listed www.rarefindnursery.com as a source. I would love to try some, but I hate to pay $25 & up, not knowing for sure whether they'll survive here. If anyone else knows a cheaper source, please let me know.

Thanks everyone. I did move the Holden, Scintillation and Roseum Maximum to a new spot with dappled shade.

I kept the Nova Zembla in its original spot in full sun -- although it is listed as sensitive to Phytophthora Root Rot.

It is planted in a lot of bark chip. To my untrained eye it looks happy so far (pic below), but the real heat has yet to start. If it struggles this summer, I will move it in fall.

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Thank you for the advice!

Many people in the South think that bark dust/chips are essential to prevent root rot. I think most of them are using pine bark or fir bark. I haven't had to use it and have no experience with it except in the pots of plants I buy.

Research in the north showed that sphagnum peat moss had a preventative effect with root rot. Now, people in the South say that peat moss doesn't work for them and in fact makes the problem worse. They are using bark dust or chips, no peat moss.

Here in the North, I mix peat humus with my soil. My soil is fairly good, not much clay.

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