The pardoning laws in the US are a disgrace, in my view. No President should be able to overturn the decisions of a court of law and the judgement of a panel of an accused person's peers.
That said, in my view, this power should only be used to pardon an individual whose case has been proven in some way to be a travesty of justice and/or the sentence passed is believed to be, by the vast majority of the public, unreasonably harsh or disproportionate to the crime.
Finally, in my view, no President should ever pardon any individual who is or whose crimes are directly associated with that President either during his time in office or prior to attaining the office of President.
In all likelihood, talk of pardons is significantly premature. They won't happen, if at all, until after either President Trump completes his second term in office or after he is defeated in the 2020 election. The simple truth, in my view, is that if President Trump were to pardon any of the individuals named in the Mueller investigation, he would trigger a Democrat push to impeach the President and such a move may gain significant support among a section of the Republican congress.