Neither the tile nor the grout is totally waterproof, and therefore, yes, you will get some moisture to penetrate, especially when hit with the shower spray directly. Now, how far in, or how much will depend on how often the shower is used and the type of tile and grout, and the quantity and pressure from the head(s). How quickly the shower dries out can be a factor - the dry desert verses the FL coast, and how well the ventilation is done (often, how long the fan is left on, or if the doors or curtain are left open).
There is no industry standard for your second question. Some like to leave gaps to make it easier for moisture to escape, some ignore that entirely. If the shower is built properly, it won't matter, at least from a damage viewpoint. IF the grout lines are not huge, and the tile is either porcelain or glazed, very little moisture will penetrate, and most often, that that does, evaporates in between uses. It's more of an issue on the pan, and unless you use a surface membrane, the slope of the liner and the operation of the weepholes are the critical things.