MonocerosRhinoceros

I may be a little late to the party here, but my understanding is this:
Tuition or fee waivers are considered financial aid and may affect your other financial aid options (including loans).  These waivers are considered taxable, UNLESS you are receiving them in return for performing your work as a grad student at an accredited academic institution.  This probably applies to most people in this forum.
 
Money received in exchange for your services (i.e. work, including TA and RAships) is considered a wage, and does not affect your financial aid.  Incidentally, this money is taxable, just like any other wages.
 
A grad stipend (as well as scholarships and fellowships), like that typically associated with a fellowship may affect your financial aid eligibility.  These funds are tax free until they exceed your educational costs.  Americans, refer to your 1098-T come tax season to see how much is tax-deductible.
 
Please correct me if I'm mistaken.  I hope that was helpful.