Fortunately, there's no left-wing media source that's remotely approaches the level of social and political damage of Fox News. I wouldn't support candidates boycotting Fox if it was merely a partisan or ideologically biased news outlet - even one with a hard-right perspective. But Fox News isn't that, currently. It's primarly a toxic mix of state propaganda, racial demagoguery, conspiracy theory peddling, and scams targeting the elderly. There's a case to be made that going on Fox can be a useful way to bring a fresh perspective to people who have become accustomed to getting their news from that source - but who could benefit from other perspectives, and who might be persuaded if those perspectives were presented in a sympathetic and credible way. There's also a case to be made against doing anything to financially benefit, or bolter the perceived legitimacy of, a disreputable and deeply harmful organization. I wouldn't fault a Democratic candidate for taking either stance.