I would be willing to contribute to this!
This is what BU has to say in terms of taxes on stipend. This probably varies but I believe graduate school websites usually outline these.
The portion of financial aid that exceeds the expense of tuition, books, and fees maybe subject to income tax.For U.S. citizens and permanent residents, this stipend is not subject to tax withholding by the University; however, it is your responsibility to report it accurately to the IRS and make payments on the taxable portion of your award.For international students, your stipend will have a taxation rate of 14% unless you are claiming exempt under a tax treaty.
from: http://www.bu.edu/cas/phd-fellowship-info/