Using the TAB face for tab-width calculation seems much likely to
preserve the local integrity of indentation (using the default face
means that mixed space and tab indentation will often not be the
same as
space-only indentation). It seems a bit more intuitive to me as
well.
However, if there is a significant case where using the default
face for
calculations is better, it would be good to know it, but it seems a
bit
silly to make decisions based on vague recollections.
I think the issue was vertical alignment of elements on different
lines,
using different faces. E.g. a first (header) line in one face,
followed by various lines in some other face.
OTOH, you can usually add a `default' face to the TABs to impose the
use
of a uniform tab size, so maybe it's not a real problem.
Maybe Gerd remembers?