ET-C will go X-D on the 31st. Payment date is 2/15.
The reason to hold through the dividend date is the reset in May. I can't fathom that ET, which is in a financial position to call C and has a history of calling their prefereds, would not do so. Even if they don't call C or do not call it right away, investors will base C's value on a call at par. So C should trade around par post May's dividend, call of no call. As the quarter progresses investors will be willing to pay above par as any call will be at par plus the quarters accrued dividend.
To answer your question there are no guarantees that C will drop by the amount of the dividend on the 31st. Certainly not May's dividend (even if no call has been announced) as there's a spread to capture and investors know this. So holding through their X-D's requires no thought. Trying to buy C post X-D requires thought and timing..
Disclosure. I do not factor tax ramifications when investing. I'm solely focused on maximizing my pre returns. My tax bill reflects my portfolios returns. I know of too many investors who sat on winning positions while they turned into losing positions to avoid a taxable event.