Preparing concrete slab (at grade) for ceramic tiles

The books stress having the slab flat, grinding the high spots and filling the low ones. But just how flat does it really have to be? Can't I just use a little thicker layer of mortar to fill the known low spots as I lay the tiles, especially if I "beat" the tiles into the mortar to assure good contact as some books recommend?
Some books and websites also stress the importance of a level slab. What's so bad about it not being level to within 1/4" per 10 feet?
My slab is at grade.
Thanks,
Ray
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*It won't be too much of a problem with 4" tile. However if you use 12" or bigger, the tile will teeter totter over the point between high and low. Try laying the tile down on a dry run to see how it works out.
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John Grabowski wrote:

If there are dips in the slab, they can be found and marked before laying the tile. Where there is a dip, just butter the back of the tile a little bit to fill the void - no teeter-totter. We have 20" tile and probably minor unevenness - none that were obvious.
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Ray K wrote:

We had two large rooms in our condo (on a slab) tiled several years ago. It never occurred to me to see whether the slab was LEVEL - good grief! If it wasn't level, it was long past the point of doing anything about it. Being an older building (build around 1975), the major concern was cracks in the slab. We tore up our old carpet and padding to make sure of what we had before we ordered tile. We found the tile we liked and then had a contractor install it. Not one crack in the slab. If there was any unevenness, it wasn't obvious - seems like we slid a level around a few places to check for dips, but nothing noteworthy.
One challenge of our job was bringing tile to the level of the terazzo floors in adjoining rooms....solution was to apply the mortar a little thicker and slant the tiles ever-so-slightly. Can't tell by eyeballing it, but the difference would have been about 1/4". The tile guys did a perfect job, even with several challenges.
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