I am new to the nom/bower stuff, and I am not exactly sure of the purpose of this folder, so I am not sure if it is supposed to be part of your repository or not. Do you guys exclude it or let it get checked in?
Yes you should include it within the .gitignore :) All of the packages pulled into their have their own .git repos, so no point including them in your own.
I'd put it the projects .gitignore file, just like the vendor/ folder.
Since it isn't a OS specific folder (like .DS_STORE etc.) it shouldn't be in the global .gitignore file in my opinion.
If you're building a project that only you work on, then sure put it in the global ignore file. But what happens if your building a project shared with others? You cannot guarantee they have /node_modules in their global ignore file as well?