There is no clear provision in Income Tax Act on this issue. TDS on Service Tax implies tax on tax and sometimes the Bill amount without service tax may be below the taxable limit . However , as prudent practice ,TDS may be deducted on the Gross Bill including service tax.
There is recent clarification given by the CBDT on the very same issue and according to the same Tax is to be deducted on the whole amount (incl.: service tax or any other taxes)