Although the Pin Top seals the bottle when screwed tightly, there is a chance that liquid can seep out the tip and pool inside when the bottle is horizontal. Liquid won’t leak out of the closed cap, but it could be messy when the Pin Top is removed. Storing upright is really more important when using the Medium tip because the pin doesn’t fit quite as snugly as it does into the fine tip, and if the bottle remains standing while capped, the leakage potential is eliminated.
It may not really be necessary–as we all leave the bottle on it’s side lots of the time!
