I only use -g when installing runnables, like less, jade etc. I try very hard to reduce the number of CLI apps that I install this way (I've written several build.js scripts that use less/jade/etc as a library instead of an application) because I don't trust the npm repository (no signing, no guarantees, etc).

Please avoid -g unless installing a CLI tool. You will only end up in version hell if you install globally, which is what node_modules was designed to avoid.