What - if any - annual maintenance is required for grout/caulk in a shower?
The surfaces that the tile are mounted to can shift with respect to one another over time and through the seasons. This can be caused by temperature changes and by general "settling in" of the house itself. 

The nicest way to reseal the cracked grout is to scratch out some or all of the grout in the cracked areas and reapply new grout. Having said that this can be more than a simple 5 or 10 minute task. It is also possible that further movement is possible of the tile mounting surfaces and your re-grouted joints may crack again. 

Caulking with a silicone based sealer can be a quick and easy way to cover up the cracks. Silicone will stay flexible and will permit additional movement of the tile surfaces while still staying sealed up. Now with that option put on the table it is up to you to decide if the appearance of the caulk sealer is acceptable to your tastes and aesthetics.