That is not mandatory for a TF-ian. However, during the year there will be many TF initiatives which will need funds to be raised. A TF-ian is expected to contribute any amount in any of the fund raisers according to his capacity, by not overstretching him/herself financially.
At the same time, please note that a TF-ian can help other ways too. Like blogging/posting in TF site, taking part in planning, execution of TF activities, donating in kind (not cash) to a beneficiary etc.