As it has been some time since I have had any experience with this sort of thing, it is unclear what exactly you mean by "certificate spoofing". However, from my understanding of MITM attacks, the implementation of the more secure HSTS protocol used by Google et al. mitigates against many of the attacks using methods such as sslstrip. I'm not sure if banks also employ this protocol, although they most certainly should.
Moreover, the addon HTTPS Everywhere comes standard with the TBB and you may want to review your configuration of this addon if this is a concern.
I'm not sure both of these implementations in tandem obviate the threat you speak of, and I'm not entirely sure that they protect you from quality forged certificates bought on the black market.
However, you may wish to the assess the strength of your own bank's SSL implementation at the Quallys SSL Labs website.
The above concerns notwithstanding, I believe that this comment of yours may be the far more relevant reason as to why you shouldn't be accessing your online banking account.
I am somewhat new to Tor and am now using it for everything.
The purpose of Tor is to protect your anonymity, something that is completely undermined by your use of any account linked to your real-world identity. If some person were to connect to, say, your Facebook account using Tor, then we could be almost certain that this hitherto anonymous individual was actually... well, none other than you.