How harmful are hairline cracks in ceramic tile on my shower wall?

My ceramic tile shower is around four years old. There are ceramic shelves in two corners of the enclosure and I'm guessing the shampoo and body wash bottles on the shelves were too heavy. There's a tiny crack under one edge of one shelf, and there is now a 2 to 3 foot long crack running down from the ceiling around a foot away from the other shelf. The crack changes direction and goes diagonally a few inches toward the shelf.

The cracks are nearly invisible and I don't think they detract from the appearance of the shower, but I'm concerned about any possible additional damage. Is it all right to leave them or should I get them repaired? If they need to be repaired, how urgent is the problem?