Blueberry shrubs benefit from soil amendments, mainly vegetable fiber such as compost made from leaves, or peat moss. Our native soil has moderate clay content, but it does drain OK. I do not see evidence of water accumulating around the roots of the blueberry shrubs in our yard. I suspect that if we had poor draining soil with very high clay content, we would have to use raised beds for growing blueberries.

I've got very sandy soil. I live right by the Mississippi river, so about a foot down its almost pure sand. While i didn't add any amendments when planting my trees, i've always had a nice layer of mulch about 2ft around the tree. This seems to help a lot. My trees have thrived with almost no extra watering...and we've had dry spells over the years (and some very wet periods too).

I worry about textural difference and capillary flow. Amendments completely surrounding the roots of a different texture than native soil can create problems, either making the planting hole too wet or too dry.

If the soil is inadequate I will amend a fairly large area with stable organic matter like leaf compost or composted stable waste, mixing it very well with the top 5 to 8 inches of native soil. By amending the top layer well beyond the roots, say 6' diameter circles for a tree with a 3' diameter root spread, I encourage the rapid development of feeder roots outward.

If this is done in conjunction with loosening the soil outward to the depth of the root ball or roots so growing roots have well aerated soil with a rich loamy top, trees will usually establish very quickly.

If the soil is already of good quality and reasonably deep this isn't needed but I plant hundreds of bearing age fruit trees and it's very important that they establish in the best possible conditions.

I'm not going to get into this much because I don't have the time to write a lot or to check back frequently (it's raining today, so I'm inside), but...

1. Review the recent literature. Do the studies show a benefit to amending soil, even sandy soil? Basing what you do on gut feeling or opinion may not be as reliable as using scientific methodology. Horticulture has come a long way over the years, and not everything that people thought they knew has turned out to be true.

2. As you indicated, there is no "THE way to do it" because there are many variable involved and different situations present different sets of variables. However, I'm not sure I would characterize it as containing a "vast numbers of exceptions" either. I guess what I'm trying to say is, that even though there are definitely situations where amending backfill may be beneficial, it's practice is still much more widely practiced than would be indicated by research. I always chuckle a little when I remember a participant of a discussion about drainage saying, "It doesn't work that way in my part of the country."

3. Amending backfill can be detrimental to drainage (or beneficial, in special circumstances) and detrimental to root growth. While the backfill may not block root growth from the surrounding area, it can encourage root growth to be much more concentrated in the backfilled area (leading to a poorly formed root system).

Just some points to consider:

Not helping if far from hurting, the latter being the usual reason given against amendments.

In very sandy soil that cannot be watered frequently (say you can get to it just once a week or longer), a newly planted tree will very much benefit by water-holding amendments. (My situation now.)

Horticultural researchers and extension personnel specifically recommend adding moss peat (Sphagnum) to blueberry planting soil (as noted by another person, above). The way they advise to do so makes the practice essentially half way between amending a planting hole and amending the surrounding upper soil zone (the certainly desirable practice mentioned by another poster, above).

Sometimes one might like to encourage initial root growth downward rather than outward. Again in my case, getting roots quickly to a depth where moisture is more available would be a real benefit. If I had a power auger there would be a 10-15 foot deep 4-8 inch wide amended column to depth under each planting hole. If I had the cash for a lot of peat that column would have so much of that intermixed as to help "wick" moisture up in addition to holding moisture against downward drainage.

I think general advice against amending is probably overly simple and sometimes misplaced.