Although the University of Pittsburgh is not able to give official tax advice, below are some resources for tax information relevant to postdocs:
(1) Information of University of Pittsburgh Postdoctoral Classifications, Postdoctoral Earnings, and Postdoctoral benefits can be found here:
http://www.oacd.health.pitt.edu/sites/default/files/PDFs/Pitt Postdoc Classifications and Benefits June 2012.pdf
(2) Information about the Pennsylvania tax code as it pertains to postdocs can be found here (see example #5):
http://www.pacode.com/secure/data/061/chapter101/s101.6.html
(3) Information regarding taxability of NIH-funded stipends (NRSA grants, like F32s and T32s), which applies to Postdoctoral Scholars, can be found here (under section 11.2.10.6):
https://grants.nih.gov/grants/policy/nihgps/HTML5/section_11/11.2_individual_fellowships.htm
(4) A general Overview of Tax Issues for Postdocs can be found at the National Postdoctoral Association (NPA) website here:
http://www.nationalpostdoc.org/?page=TaxIssues