I saw only part of the news conference today, and so I don't know if anyone asked Trump about his tax returns.  He should be asked about the tax returns in every press conference and interview until he either produces them or is no longer president.
Before he ran, Donald Trump promised, without conditions, to release his returns if he decided to run.
He has not produced the IRS letter notifying him of an audit.
In the unlikely event that his taxes are really being audited, this is not a valid reason to withhold them. Also, even if they were under audit, we don't know and have no way of knowing if the audit is ongoing.
Donald Trump is lying about his money: about how much he has; where it comes from; where it goes; whom he owes; who owes him.
He's lying about his relationship with Russia and with Putin. The key to understanding those relationships may be found in the tax returns. We won't know until he releases them.
He should be asked every time he is in front of a microphone.