We used 12X24 size tile in our bath. Not sure if tests for good quality porcelain is the same for all sizes. But our tile guy put one of the porcelain tiles on a flat service (I bought a sample) - glass topped table - and he looked at whether the flatness was even - the corners seemed to be particularly important. I think the main point he was looking for was minimum "lippage" - so if tiles are flat on all sides - that means you can have a very flat wall - something we wanted. If you have home made rustic tiles (or something like that) - I don't think you would care about flatness but then they probably wouldn't be porcelain either.
I'm not a tile guy so take what I'm saying with a grain of salt (kind of) but this is what I saw him do for testing and this is how I understood what he was looking for.
The end product for our porcelain tile wall is VERY flat which has a very nice quality look, IMO (though I'm a little biased.)
I don't know anything about the tile shop. I do know my tile guy requested that I shop locally for tiles so he could actually make the purchase and inspect the boxes before accepting the product at the store/warehouse.
If your tiles aren't totally flat, there are methods for laying the tile that can minimize the fact that the tiles aren't flat on all four corners but I think I'm done with everything I know on this subject.
There are tile forums such as John Bridge where you could get more precise info from some experts. That's one place where my tile guy hangs out and his work is very high quality.
p.s. our 12X24 tile was very cheap, $1.99 sq ft from Floor and Decor - they get left overs from some quality tile sources so cheap doesn't always mean poor quality.

This post was edited by elphaba on Wed, May 1, 13 at 1:39

Thank you elphaba for your feedback, I will check this site . Any way my turned out ceramic not porcelain , so wan't work for the floor.

I visited The Tileshop, they have a store in my city. I would have no problem ordering from them.

The quality of the tile appears to be the same as other tile stores I visited. The thickness and overall appearance is the same and I'm sure they would hold up just as well.

They even have some of the same tile as what I saw at Ann Sacks. The only difference is in the glazing/color. I believe the Ann Sacks tiles have better glazing (my guess is they have the manufacturer run an extra pass or two to get finer detail on the coloration).

I was just there today , and picked up a tile that I liked . It looks like a good tile made in England . However when I came home and compared it to the other porcelain sample that I got from another store , the new one weighs much less than the old one . The thickness is different too . Would weight be a factor in the quality of the tile.

See my other post that includes a porcelain marble image I inserted. I talk about my thoughts on the Johnson Tile.