This was common wisdom back when houses weren't insulated and walls, doors and windows were drafty and porous to humidity changes.  If your house is modern or updated to code there should be no problem with placing a piano on an outside wall.  There are a few factors to watch out for when placing a piano - avoid direct sunlight as much as possible as UV light will fade color from a finish over time, and fluctuations in heat will affect the tuning somewhat - though not as much as humidity changes will.  Also avoid places with moving air such as heat registers and proximity to outside doors - especially if they tend to get left open during warm weather. Another thing to watch for is fireplaces or woodstoves used as a heat source - try to keep pianos as far away from direct heat or blowing air as possible. If there's no other option than to place a piano  in a non-optimal position there are products such as covers to protect from UV  or piano specific humidity systems to help stabillize the tuning.  Feel free to call and ask about price and availability...