the most important points in laying out your structure are: be MECE, be quick, be sensible. The breakdown that you are suggesting is a deeper layer that might not even be evident at the beginning of the case. Therefore I would suggest, as a rule valid on average, to bring it up only ahead in the case.
When the time to dig more into each element of your profitability tree comes, it is then a good idea to suggest that there are a number of dimensions you might use and then to pic the one that makes more sense in the specific situation.
However, each rule as exceptions and it might happen that the segmentation is easily understandable from the beginning. In this case, you can mention it with no disruption in the MECEness or speed of your structure. For example, if the case states from the beginning that there are two products originating income, you can already segment the revenues in 2 sub-branches correspondent to the products. Later on in the case, it might be necessary to create sub-brunches, splitting for example each product into different customer-categories. In this case, drawing a matrix might be a good idea to impress your interviewer.