Here’s a matter of style. And while I welcome opinions, I’d also like to know if there’s an authoritative, “definitive” ruling anywhere!
Say you have a table where some columns have a simple column head, and other columns are grouped and have span-head with subheads underneath. Like, for example, this:
Now, should that single-column header be aligned with the top row (as per this example) or with the bottom row? My manager (and, it seems, our typesetters) think the former. Many of our authors seem to think the latter. I can see both sides of the argument: aligned at the top seems to make structural/semantic sense, but aligned at the bottom keeps the heading near to the data it relates to.
Example tables in Chicago and Oxford style guides seem to side with our authors, but they don’t explicitly talk about this issue.