Is that the case in the US? My impression is that Ford gave Nixon a blanket pardon even though Nixon had not (yet, probably) been found guilty of anything.

You are correct: http://www.factcheck.org/2008/06/blanket-pardons/

A promise of a pardon would have done just as well, especially for federal prosecutors. The DOJ isn't going to make a big case out something their boss has promised to throw out. That said, the president being superinvolved in an ongoing case like that would be very strange.

I assume you mean convicted, but Ford gave Nixon one.

Actually I do mean guilty, as I was under the impression that you have to accept your guilt before you could get a pardon.