I was asked a question yesterday, is there a particular order in which to set the mosaics? Yes, absolutely. In Roman mosaics it is important not just to lessen your chance of making mistakes but also it's important because you need to learn certain skills. You have to develop your 'eye', your ability to see the spaces and to work within the space that you have.
I'll post up a bit later to explain this for both figurative and geometric mosaics.
Here's the article, go to this page here