Tufte has a lot to say about this (and many viz things) - worth reading his books, really. The general guideline I try to remember is "facilitate comparison" (present things that you want the viewer to compare/see near each other) and "maximize data/ink ratio" (put as much data, and as little other stuff, on the page). The latter, in particular, tells us to reduce lines from tables as much as possible; I often find just one (not the three this approach recommends) to be plenty, depending on the data. That is, just the line separating header and data. Of course, YMMV :)
