I'm not adding a perfect analogy to Borek:'s initial one, but consider the $10 bill in the shredder. If you shred 3/4 of it, you don't have any money left. If you shred 1/4 of it, you do, people look at it funny, but still accept it. If you shred 49% or 51%, you may, or you may not, depending on the person who might accept it. If you're going to perform a titration, you will have to account for decomposition of the solution, it all depends on how much is gone over what period of time, how accurate you need it be, how recently you've checked, etc.