Yes, it would cause you to slow down, but if you are doing down hill you are gonna want to slow down anyway(depending on the hill). This is really a case by case thing because if the hill is tall or long enough this will save more gas than if the engine was idling down the hill. The gas savings of one method over another are negligible when talking about a bike probably but in a car it is much higher since the the heavy car will keep momentum and will not stop as quick. There is a hill I encounter on my way to work where if I leave my car in 4th while going down the hill I max out my "Current MPG" gauge at 99.9mpg and I can watch my "miles till empty" gauge increase 2 to 3 miles since I am not using gas for probably half a mile. However, when I put it in 6th gear on the hill my MPG is around 50 because the engine is actually running slow enough it needs gas to stay running and I am using my brakes a lot more which is wasting energy. This method is also how you should slow down as well. Instead of shifting into neutral and coasting to a stop, leave it in gear and as the engine slows down, down shift to the next gear and so on until you are stopped. This will not only increase the life of your brakes, it will give better gas mileage and I believe it is even illegal to be in neutral when moving(unless in an emergency situation such as hydroplaning or something) because you need to be able to react to things in an emergency.