The following picture shows two different ways of using standard cross style (+) plastic tile spacers.
Using them in the plane of the tile as is seen in the bottom left and right corners is the "dumb as a bag of hammers" way to use them. People look at their shape and immediately deduce the wrong way to use them. And, some web sites will tell you to remove them before grouting (which is much easier said than done) while other web sites will undoubtedly tell you to grout right over them.
The smarter way to use them is with two spacers between each pair of adjacent tiles as is shown. That way, you can easily remove the spacers before grouting.
I can't say about floor tiles, but my experience with wall tiles is that the tiles are NOT all identical in size. You do get minor variations in size between tiles that would make using cross style (+) spacers at the corners problematic. If you have 5 or 6 tiles in a row that are all ever so slightly wider than the other tiles, by the 6th tile, you can't get the spacer into that corner any more. If you use them as recommended, you can compensate for slightly wider tiles by turning the spacer to get a narrower grout line width. Just by orienting the spacer one way or the other (perpendicular to the grout line or parallel to it), you can create a small difference in grout line width because the spacer arm will be slightly thicker than it is wide or vice versa. They do use a different kind of clay to make wall tiles, so that may result in greater size and shape variations in the baked wall tiles as opposed to floor tiles, I don't know.
By default, try to inserting your spacers to obtain a slightly wider grout line. That way, when you get a slightly wider tile, you can compensate by rotating the spacers to get a slightly narrower grout line. Also, you can compensate for narrower tiles simply by centering the tiles over the row below to keep your grout lines vertical. As long as you use spacers under the tile to prevent it from sliding down the wall as the thin set or mastic sets, it's not even necessary sometimes to use spacers between pairs of horizontal tiles. Also, tiles are not always rectangular in shape. Sometimes they're slightly "rhomboidal" (if that's a word) so that if the bottom of the tile is horizontal, the sides of the tile might not be exactly vertical. In that case, you CAN'T use two spacers between tiles because one of them will fall out. There's no clamping pressure to hold it in place. All of these differences are so small as to be imperceptible when the job is done. When you stand back and look at the finished tiling, you won't see any problems, despite these differences causing you problems when you're tiling.
That is, it's been my experience that the tiling can look like a mouthful of crooked teeth (I'm exagerating a little) when you're tiling. But, once you grout, then the continuity of the vertical and horizontal grout lines restores some semblance of order to things, and the tiling will turn out looking geometrically fine. Or, lemme put it this way: I've tiled over 25 bathrooms, and in some of them the variation in size and shape of tile was really starting to worry me because I was concerned that people might notice the vertical grout lines not lining up altogether well in places. But, once grouted, it all looked fine, and I've never in 25 years ever had a tenant or even a prospective tenant so much as comment on any tiles or grout lines not being in good alignment or agreement with the others.
