The three positions for storing wine bottles are horizontal, vertical and tilted. Choosing one among the three positions depends on various factors.
If you are storing your wines in wine racks and is intended for long term storage, horizontal may be the best position. This will keep the cork moist and swelled and prevent wine from unwanted contact with air or air pockets. Also, the sediment will be deposited on the sides of the bottle, which will allow clear wine to flow when wine is poured. So, this is the perfect storage for ageing wines.
If the wine is meant for short term storage or dining occasions, vertical storing is preferred. It is the same case with wines that are consumed while young. It will help in easy viewing of the labels. But a negative to this kind of storing is that it will dry out and shrink the corks, which lets in air into the bottle and thereby spoilage of the wine. Also, the wine quality may be compromised because of the pieces of dry cork that crumbles into the bottle when being removed.
If storing wine bottles in tilted position, a slight upward angle is preferred so as to keep the cork damp. If it is too upward, this position eventually dries out the cork. The only advantage that is seen is that the sediments are gathered at the bottom. If a head-down position is chosen, the cork remains moist but it allows sediments to accumulate near the cork. A periodic rotation of the bottles for ensuring moisture on all sides of cork, is required in addition to a proper tilt, when storing wine bottles in a tilted position.