It's common for Justices to recuse themselves from cases in which they were involved before becoming a Justice and I was wondering if Taft ever had to do the same.
The case histories of the supreme court are well-documented in the United States Reports
So far, I've found that none of the cases the Taft court heard had at issue laws that were passed under Taft as President. Constitutional challenges of federal laws (those that we would have signed), which are less numerous than state laws as typically more difficult to challenge, are less common in the court's history of cases tried.
I can find that Taft recused himself three times:
Lucas v. Earl, 281 U.S. 111, 50 S. Ct. 241, 74 L. Ed. 731 (1930)., Concerning the Revenue Act of 1924.
Corliss v. Bowers, 281 U.S. 376, 50 S. Ct. 336, 74 L. Ed. 916 (1930)., Concerning the Revenue Act of 1918 and 1921
Both of the above cases were decided AFTER Taft had resigned the office because of his declining health, but still they bear the mark of his lack of participation without other notation.
This is the only case I can find that he chose to recuse himself voluntarily.
An interesting side note is that although you describe the practice of recusal as 'common', and indeed today it is, the ethical standards of recusal in American jurisprudence were not always as codified and widespread. That's not to say that recusal was unusual or that the ethics behind a fair legal proceeding were any different, it's just that there was little in the way of legal authority to insure against such conflicts of interest.
It was Taft, just appointed to the Supreme Court, working with the American Bar Association to draft the first version of the ABA's Canons of Judical Ethics. This publication wasn't binding or otherwise legally enforceable, but it was a persuasive and influential set of standards that set the starting points that later and more objective canons would be based upon: The better codified and more enforcable ones that inform the 'common' ethics of jurisprudence today.
Yes, I know for certain that Taft recused himself from Biddle v. Perovich (1927) which dealt with the issue of presidential pardons. As President, Taft had commuted the sentence of Vuco Perovich from death to life imprisonment. As a result, when Perovich's application for a writ of habeas corpus was presented to the court, Taft recused himself.
Why was Perovich asking for habeas corpus relief? Well, when he learned that Taft had commuted his sentence from death to life imprisonment, he pushed for a pardon. Perovich argued that his removal to the federal penitentiary at Leavenworth was without his consent and without authority, and asked the court in Kansas to release him.
The question before the Supreme court was whether or not the President had the authority to commute the sentence of death to life imprisonment. This is notable because not only did it concern the powers of the office Taft had held, it also directly concerned an action he had taken while holding said office.
The court ruled that the President may commute a sentence of death to life imprisonment without the convict's consent. The constitution (Article II, Section 2) expressly grants the President the authority to "to grant reprieves and pardons for offenses." Whether or not Perovich wanted or accepted that mercy was irrelevant to the constitutionality of the President's action and therefore the writ of habeas corpus was denied.
Source: Biddle v. Perovich, 274 U.S. 480, 486-488 (1927)