The best mulch for rhododendrons and azaleas is conifer bark. It breaks down slowly and maintains soil acidity.

Mulch products have become big business and you do have to be really cautious about what you're getting. A great deal of the stuff sold as mulch consists of ground up and dyed construction and other debris. So called hardwood mulch is usually the by product of a manufacturing process and is treated in a similar way. Hardwoods contain a much higher percentage of cellulose in both the bark and sapwood (which probably is 90% or so of the total content). This means it breaks down much more quickly. This can cause nitrogen deficiency and pH problems although neither occurence is terribly likely when used as a mulch as opposed to a soil amendment.

Scientific studies have shown that the net effect of different mulches on the pH is negligible. There is a small temporary effect where most all conifer derived mulches do lower the pH slghtly.

When any wood rots, it robs the soil of nitrogen. But as aka mentioned, that is not a significant problem either with mulches. I have a friend and rhododendron hybridizer that uses sawdust from a maple spoon factory exclusively to mulch his rhododendrons. People come from across the country to visit his garden. To top it off, his garden is in what used to be a corn field on the top of a hill. It is fully exposed to wind and sun. His hybrids are, needless to say, very hardy. For rhodophiles, his name is John Doppel and his hybrids are available from W.W. Nursery in Indiana, PA.

I would say that the artillery fungus problem with hardwood mulches is much more severe than any pH problem. The artillery fungus can damage the paint on cars, houses, fences, and anything it gets stuck to.

That's definitely what I wanted to hear! Thank you very much for your very thorough answers.