Willy says, "I never told him (Biff) anything that wasn't decent." Is this true? What does this say about Willy?

I think there are several ways you can think about this.  If you emphasize the TOLD, then this could be an excuse for what he showed Biff by his behavior.  From here, I think it depends on how you read/judge Willie.  Willie had a lot of dreams that weren't "reality based." Biff  knows that his father had "all the wrong dreams" and that he passed them along to his sons.  Charlie notes, however, that a salesman has to dream. 

Are we supposed to blame Willie for his "dreams"?  Should he have taken the job that Charlie offered him?  Did his pride get in the way of what he should have done?  

I'm not sure.  We all have dreams that turn out to be less than we had hoped.  We also reach a point in life where we know that we have made choices, and that there probably isn't another way (think of Frost's "The Road Not Taken").  this is where Willie (to me, at least) approaches part of the definition of a tragic hero in that he inspires fear and pity in all of us who know that we can make judgments that won't work out for us in the long run ... we pity him, and we fear that we might follow a similar path.

Said in another way, "There but for the grace of God go I."