At a density of 67% solids, syrups are on the edge of growing crystals. That's one element of flavor intensity and if crystals grow it tells you the syrup's flavors are at their max. If you choose to warm syrups before serving (the microwave is fine) you will be forcing off a little more water. Each time you open the bottle, a bit of fluid evaporates and the syrup will thicken over time. To avoid crystalization, every now and then put a few drops to a 1/4 teaspoon of tap water back in the bottle. The amount of water to add will depend on remaining fluid volume and whether or not crystals have already formed. This is your judgement call. If the syrup clouds that is simply the nutrients potassium and calcium titering out of solution because it got too thick. This rarely happens and is not a problem; it's just something you'll never see in a plastic jug or metal can!
Maple syrups are non-hazardous and non-perishable, which means they do not age or go rancid like wines, vinegars and oils can over time. Dense minerals, like salts and sugars, maintain high osmotic pressure. Microorganisms cannot operate inside such fluid and dessicate on their surfaces.
Therefore, like honey, refrigeration for syrup is an option. We typically choose to leave them out on our counter. Our farmhouse syrups are by the stove with oils, vinegars, peppers, salt and other condiments as we like them handy for use.
Sometimes syrups will surface mold--a light colored film or "dotting" that forms on the top of the fluid. This may occur after weeks or months or may not happen at all. We sometimes notice it is more common with darker syrups. Surface molds are harmless and will not mix into the fluid. We choose to ignore it, simply skimming it off knowing it will not affect flavors. It can be unsightly. If you prefer not to deal with this, refrigeration is advised. In any event, never throw syrups away because a mold formed. They are fine and the molds can be removed. You can pour out the syrup into a pot, bring it to a short boil, stirring constantly, cool it and skim off mold.