Some of our ground has been grid sampled for about 15-18yrs here. Here is what I know.
When we started we determined trip levels for P and K. The P trip level is 50 and I can't recall what K trip level is.
So when any one grid tested above these levels no fertilizer was put on in those grids.
Yes/no  it has saved a considerable amount of money over those years in P&K but alot of money went towards lime instead. which would have been put on regardless so my main answer on savings is yes.
Over the course of those 15 yrs some of the ground that no P has been put on is starting to get below 50 and we have resumed putting it on those grids.(same with potash) There are some grids (old dairy) that will never need it in my lifetime..(100+ P levels) At this point in time it is still cheaper to have grid spread, But if it gets more costly than just mass spreading we will have to consider all options at that point in time.
So long story longer, yes some of the fields are getting a little closer to being spread like the old days but not very many.
This is what has werked well for me HERE, and email is in profile if you have any other questions .
I hope this has been some help.
Steve