...I have always found manufacturers recommended times to need very little alteration- a minute at most, and always *more* development than recommended - to achieve normal development based on a zone VIII.
I have not found this to be true at all; Usually, in my own tests, the mfr's recommended ISOs tend to be over-rated (thus, the advice to decrease ISO is probably well founded) and that when adjusting for that, the development time is often a bit shorter. I think what is going on here is that using their lab tested ISO, the development time must be a bit on the long side to compensate in the higher values for the effect of underexposure. A one stop over-rating in ISO is not at all uncommon.
It is not that they are "wrong". It is that their tests are based on what it takes to get the film to move, rather than what we need for a practical printing threshold. Work in the field generally proves more successful with a bit more substance in those thin areas.
In saying this, I must also qualify that film testing is dependent upon a number of factors which are extremely hard to quantify from one worker/lab to another. Equipment and working procedure variables make significant differences. What I find may not accord exactly with your results. It could vary by quite a bit.
The point about what it means to "expose for the shadows" in 2F2's post is really important in that to do so, it is not enough to just point the meter into the shadows and go with it. What's important is that the shadows are exposed correctly to be shadows, not to be mid values. Generally, adequate detail in shadows would result if, when you do point the meter into the shadow, you don't give the indicated exposure but two stops less. That is, if the reading you get in the shadow is say, f/8@1/30, the actual exposure that would accomplish the appropriate exposure would be f/8@1/125. This will produce a great deal less density in that area, but if all's well, enough separation in the densities to show adequate detail while still allowing enough exposure in the printing to produce a substantial black.
It's kind of a tricky concept to get, but as you work with it, should come clear.