Both the gritty mix and the 5-1-1 mix are intended for plants in containers. I am somewhat confused by your question since you seem to be thinking of in ground planting rather than container culture. There is no particular reason why rhododendrons could not be grown in the gritty mix, but it's great weight and the need for lots of water and careful fertilization might prove to be disadvantages. The 5-1-1 would be a better choice. Again, I am referring to pot culture, not in ground planting in both cases.

As for in ground planting, a "5 gallon hole" would not be large enough for a 3 gallon rhododendron. In fact, it's very useful to not think of holes at all. Rather, make a wide, shallow depression. Use a highly organic soil mixture with lots of airspace and plant with the top of the rootball slightly above grade. Planting in pure bark, especially if it is partially decomposed, is fine, too. Make sure to spread out the roots, but complete bare rooting is not necessary. Mixing up large quantities of either the gritty mix or the 5-1-1 then putting them in the ground is also not necessary and would not work as they do in pots. Good drainage and lots of airspace are important but can be achieved much more easily with unsifted conifer bark or any other acidic, coarse organic material.

Out here some have recommended planting them in a couple bags of pure pine bark, more or less on top of the soil. They do best planted on a hill as well and in open shade or dappled sun. They need excellent drainage and loose soil. Cannot stay wet for too long, and soil should be kept cool. Our heat, humidity and a hot soil will kill them in sun.

akamainegrower, regarding planting in pure bark, would you use a mulch that had very fine particles, or would you plant directly into small bark, which has anything between fines and 3/8" mixed into it?

I understand your point about making as shallow depression for planting, but just for the sake of discussion, how big of a hole - in gallons - would you use for a 3 gallon plant?

Last question first: I can't translate into gallons (or holes for that matter). A good deal depends in the size of the root ball of what you're planting. As a (very) general rule, make the shallow depression at least 21/2 to 3 feet in diameter.

Use whatever sort of bark is commonly available in your area. It will contain a wide range of particle sizes. Coarse particles are essential for providing the needed air space, fine particles increase water retention. This actually refers to the bark available from landscape suppliers, not screened, bagged mulch although some of these products will be fine. By all means avoid the ones which are not bark, have been dyed, etc. You will need a surprising amount of bark for even one rhododendron and it's much cheaper to buy in bulk. A good many landscape yards will load or allow you to load a trailer or pickup.