The Cellformatting event is the place where you will need to do this because the RadgridView uses UI Virtualization. This means that the cells are re-used for the data as it scrolls and as values are changed.
Cell elements are created only for currently visible cells and are reused during operations like scrolling, filtering, grouping and so on. This approach improves greatly the performance of the control.
Have a look at this article
which further explain the logical vs visual structure of the RadGridView. You need to get to the CellElement property in order to set the properties that you want to. The place to do this is the CellFormatting event.
Hope that helps but let me know if you need more information