The one case in Tax Court that raises this issue is Judge Laro's BancOne, where the Court needed to value derivatives. Laro brought in an expert he could trust because, at least partially, he perceived that he would otherwise face a situation with "exotic" testimony from competing experts--neither of which Laro felt competent to compare.
Judge Mary Ann Cohen has also used court-hired experts, in similar cases where there are only had very small handful of possible witnesses. "It's not a standard procedure," she says. And, she also makes the point that, even when the Court hires the expert, "we're not handing over the decision to them."