If you notice grout cracking in corners, but the grout looks good everywhere else, don’t bother re-grouting. If the grout cracked once, it will most likely crack again. Cracks in corners occurs because walls expand and contract at a slightly different rates, or otherwise move, relative to each other.
Inside a shower, or other wet location, make sure the area to be repaired is completely dry before you begin.
If you have white tile and white grout, a bead of white tub and tile silicone will fix the issue, keeping your shower watertight without blowing the looks of your tile installation.
If you have colored grout the fix is almost as easy, but requires an extra step in order to hide the crack and make the joint waterproof. Pick out a tile caulk that matches your grout and run a thin bead up the corner to fill in the crack. Unfortunately tile caulk doesn’t hold up well if it repeatedly gets wet, or stays wet for extended periods. No problem. Allow the tile caulk to dry overnight and then apply a bead of clear, tub and tile silicone, completely covering the tile caulk. Allow the silicone to dry overnight and you’re back in business with joint that looks good and is waterproof.
Of course if you’re problem is in a dry area, then a matching caulk alone will be adequate. The flexibility of caulk allows it to stretch with the wall movements rather than cracking.