The answer will depend on how long you plan to store your wine. Stores which don't keep their wines cool can use the excuse that the wine isn't there for very long (or isn't supposed to be anyway). Unless exposed to extremes you aren't damaging wine storing it at 75 vs. 55 degrees. However the wine cannot be kept for as long at 75 than it can at 55.
My understanding is that cycling the temperature can be harmful, i.e. you don't want to leave it somewhere that gets direct sunlight during the day because it will warm up, and then cool off at night. Likewise keep it away from a heat duct.
If you have a higher end Cabernet/Bordeaux/Brunello etc. which will not reach maturity for a number of years then you need to think about the storage temperature. But if you bought a few bottles of chardonnay to enjoy over the next couple weeks or months don't worry about it; just keep it someplace with a relatively steady temperature, and cool it (whites more than reds) before serving.
You can find a lot more details at various sites, including this one: