Is it safe to do bank transactions through a rooted Android Device having a custom rom?

"The only secure computer is one that's unplugged, locked in a safe,and buried 20 feet under the ground in a secret location... and I'm not even too sure about that one"

Lets clear one thing. No device that is connected to the internet is completely safefrom malware. Not even Linux and Mac OS X machines. What you should be asking is whether Rooted android devices are safe enough for bank transactions.

Now answering your question ,  it depends on what customisations have you done and how you have used it.

One of the major downsides of Rooting is that it greatly increasing the risk since you are breaking the defence mechanisms of the OS. Rooting only increases the security only when you have advanced knowledge in OS security and know precisely where and how to plug the holes. You should also seriously consider you track record of online activity.If you visit only trusted sites and install apps only form trusted sources , you should be fine.

Another factor is the authenticity of the ROM you are using.If it is something like Cyanogenmod or Resurrection Remix (downloaded from their official sites) ,then it is as secure as it can be.Do not attempt any online transaction if you 100% trust your ROM developer(s).As Ahmad Nabeel said , a preloaded key-logger could be present in the ROM. Key-loggers track whatever you type using your soft/physical keyboard which includes passwords.

Since you have installed a custom ROM , I believe you must have an unlocked bootloader. Keep in mind that any one with physical possession of your device can boot it into an unsigned image which can be used to steal your data regardless of password set or not. In some cases it is also possible to break into your device even if its encrypted . So keep your device safe with yourself.

Summing it up , if you think that your ROM is secure , give SuperUser permissions to apps carefully only after validating their authenticity (some root apps have backdoors installed and when a malware gets root access , you are screwed) and visit only healthy sites , go for it.