In general I would not suggest regular habits of adding keys to help taper notes. In some very specific circumstances, maybe, but not as a general rule of use.
I'd highly suggest picking up a copy of Arthur Weisberg's "The Art of Wind Playing". A great deal of the book deals with exactly the kinds of things related to the kinds of problems you're having. It's a bit dense though, and I don't know your age or level at this time, so it might be an inappropriate suggestion. Still a good thing to have in your collection.
The gist of it though is that as you reduce air flow you need to compensate with embouchure in order to maintain pitch. It's a tricky balance, but what's odd is that usually we tend to pinch the reed in order to go quieter and as a result the pitch would go UP, especially on the open F. Keeping the balance all the way until the end of the air stream is pivotal. Weisberg describes it much better in his book.
M.M.A., D.M.A. University of Illinois at Urbana/Champaign: B.Mus. Lawrence University
Bassoon professor at University of Wisconsin Eau-Claire
Maker of the Little-Jake electric bassoon pickup and Weasel bassoon reeds