Does ATtiny RESET pin need a resistor?
First, in the ATtiny25/45/85 datasheet, page 161, it suggested that there's a internal pull-up resistor between 30 kΩ - 60 kΩ on the RESET pin. It means the MCU will almost always work for a hobby project. However, if the requirement is high reliability, 


  I need to make sure that my ATtiny does not reset unexpectedly.


Then you have to put external EMI/RFI and ESD into your consideration. I strongly suggest reading AVR040: EMC Design Considerations.



Finally, it should be noted, that unexpected reset is inevitable for any MCU facing hostile environment in the long run, sometimes you even have to trigger it deliberately (e.g. through a watchdog timer) to prevent damage from a runaway program. On one hand, you should take precautionary measures in your circuity to defend yourself from spurious reset and ESD, on the other hand, you still need to make your program to be robust enough to handle unexpected reset.