I've seen web sites that use the MS convention of using the "+" (plus) and "-" (minus) sign to expand and collapse rows of folders. On other web sites I've seen the Macintosh style of using the little triangle (arrow) pointing to the right to show a collapsed row (nested folder) and pointing down to represent a folder that has been expanded.
Question: Is it designer preference or is there some web design convention that is most common?
Designer preference.
Amusingly, if you wished, you could check which OS the visitor is coming from and display the appropriate image for them :)
To be honest, I've never really noticed a difference, they both seem quite logical and reasonable to me.
Windows uses triangles these days (Vista and 7).
A triangle pointing to the right for closed folders and one pointing right-down (so that the point of the triangle is in the bottom-right corner)
I'd say go for triangles, if it's what users are used to then they will find it more usable