Think of it this way… glass is best cut in straight lines. If your wall is not plumb or your threshold isn’t level, that’s OK. We can have the glass cut to correspond to the slope of these surfaces—as long as they are straight.
The upshot is that if the tiled surfaces upon which your shower glass rests is not “straight,” that tile needs to be straightened. The most frequent example is when rope tile wraps around the shower enclosure horizontally and obstructs the placement of the glass—notably the shower doors—against the wall. We can notch this out with our diamond bit grinder, but it would have been better for your tiler to stop it short of the “footprint” of the shower glass.
But if your tiler has placed a beautiful granite shelf which has an overhang in a place over which the glass of your shower enclosure must run, your tiler usually has to be called back to pull that shelf out, cut it down so that it is flush with the tile below it, and re-mortar it (the usual choice of homeowners) or your shower will have an unsightly gap next to the wall beneath the overhang with the potential for leaks.