I think we're all tempted to use that very last bit of floss just because we hate the thought of ending, rethreading, starting and ending again. I would throw this advice out more to new stitchers rather than experienced ones who probably already are aware of it. If you are down to the last couple inches of thread in your needle you probably should have discarded it several stitches before! As the floss gets pulled through the fabric, the doubled over part in your needle especially, gets worn and starts to thin. You will notice this more easily when stitching dark floss on light fabric and vice -versa. Other factors that will contribute to wearing of the floss is using too long a length or too small a needle. The eye of the needle should be big enough to open the hole of the fabric so that the floss passes smoothly.
Just an opinion based on years of observation. :)
Mavia