Earlier today @tomhenzley tweeted about models for teaching addition and subtraction of decimals. I've often found the 10 square and 1 square to be a useful addition to other strategies. In many classes, children are very used to the 100 square to add and subtract 2-digit numbers. It is a natural progression to use the 10 square and the 1 square to move on to decimals.
In many ways the 1 square is easer to pick up because it represents numbers in the way that children are used to see them with money. You could even call it the £ square (or the $ dollar square, or lots of other denominations of currency for that matter). The 10 square is a little bit more difficult to link to real life, although if you produced one with two decimal places you could put it into the context of timing a 100m race.
It is important to remember that number squares are limited as a image for children – number lines are so much more flexible because they are extendable and easier to produce freehand – which is important for jottings – that all-important stage between mental methods and standard algorithms.
If you want to try it for yourself, have a look at my Google Spreadsheet of the 10 square and 1 square.