what to do with JTAG pins when idle?
I would suggest referencing the specksheet for the specific device - they usually have pretty extensive documentation and try to cover most of questions. Here's what I found in my family of devices:

XC9500, XC9500XL, XC9500XV, and CoolRunner XPLA3 have internal pull-ups on TDI and TMS.  CoolRunner-II devices have internal pull-ups on TDI, TMS, and TCK.
It is recommended to place external pull-up resistors on the JTAG input pins TDI, TMS, and TCK. For a single device, the resistor value of 4.7 kΩ is suggested. This value may be adjusted depending on the number of devices in the JTAG chain or the application. External pull-down termination is not recommended as it would conflict with the internal pull-ups.