Chocolate – Is it better to store chocolate in the fridge or at room temperature

Regardless of type, all chocolate should be stored in a cool and low humidity (dry) place away from direct sunlight. It would be best to seal it in an air-tight container, because, as ElendiTheTall said, the cocoa butter in it will absorb flavors.

Dark chocolate will last for years. Milk and white chocolate will last for a much shorter time (a few months), because of their milk content.

Improperly stored chocolate will develop bloom, which shows as a white or grey streaking or spotting on the surface. The spotting or streaking is cocoa butter fat separating and is a sign that the chocolate's temper has been lost. This kind of chocolate is still suitable for any application where the chocolate will be fully melted (most baking). It can even be used as the base (non-seed) chocolate for tempering with the seeding method, but it should not be used for other candy making.

It depends on what you're doing with the chocolate.

If temperature isn't that critical and its going to be mixed into a cake or brownies or such, then do it in the microwave. Its simpler and as long as you don't rush it, its does a good job. Just go 10-20 seconds at a time, in bowl that doesn't hold heat well.

If you need precise temperature controlled chocolate (for tempering perhaps), its best to use a double boiler. The bottom pan just has steaming water - so I never really count that as a dirty dish. With the double boiler, you get several advantages. It won't get any hotter than the water. It its gradual and gentle. And you can monitor and stir as the temperature rises. If it turns grainy, its likely a bit of water dripped in. Depending on the top bowl on the double boiler, its possible to accidentally knock some condensation into the chocolate...and now its toast. I use a top bowl that has a small outer lip on it to keep this from happening.