In my experience, it doesn't make a huge difference what you do (downshifting, nuetral, staying in gear until you're almost stopped) MPG wise. It's very subtle so long as you drive well. I always anticipate my stops early and start to let the car slow itself, then I downshift with rev matching to not wear the clutch. Then once I have to use the brakes, I heel-toe downshift (again to not wear the clutch excessively) to come to a stop. I will only downshift to 3rd or 2nd depending on the conditions. If I can easily stop, 3rd - if I'm facing downhill or just need to stop quick for some reason I get into 2nd. The engine will naturally want to drop its speed, helping you in slowing down. I don't always do this, sometimes I can anticipate a stop and just gently brake in gear. My mileage never changed much. I did tests driving my same routes using only neutral and braking, downshifting and braking, leaving it in gear and braking, and heel-toe downshifting. The mileage was basically the same for all of them, only varying maybe 1 or 2 MPGS at the most. So, I'd say do what you like doing. I personally just feel more in control downshifting, because it really helps slowing you down. But some people think it's extra work.