When tiling walls you will at some stage obviously need to tile around a window or two. If you are tiling a wall in which one window is the main feature then hopefully you will have set your tiles out so that you have centred the window making sure that the cuts either side of the window are even, and if possible more than half a tile, although sometimes this is not possible as it gives really horrible thin cuts at the end of the wall. Every wall needs to be looked at individually from the point of view of centring windows against centring the whole wall.
The biggest mistake that Do It Yourself tilers make when tiling around windows is that when they get above the top of the window the tiles do not line up so that the last grout joint is too wide...or there is no room to fit the last tile in!
This can be avoided by using a spirit level (or plumb line) from the outside edge of the tiles at the bottom corners of the window to draw a pencil line straight up from the outside edges. If you then follow these lines with the outside edge of the tiles that run up next to the window then this will prevent your tiles from closing in or running out as you go up either side of the window. Before starting to tile up the sides of the window you can double check that your pencil lines are true by measuring the distance between the outside edges of the tiles at the bottom of the window and checking that it is the same distance as between your pencil lines at the top of the window.