You can let either Smart View or Excel control grid formatting.
Smart View formatting consists of such settings as Cell Styles (see Cell Styles) and Use Thousands Separator (see Formatting Options).
If you choose to use Excel formatting, then these Smart View settings are overridden by your Excel formatting selections. Your Excel formatting selections, including conditional formatting, are applied and retained on the grid when you refresh or perform ad hoc operations except Pivot.
When you use Excel formatting, Smart View does not reformat cells based on your grid operations, and it does not mark cells as dirty when you change data values. Smart View does preserve the formatting on the worksheet between operations.
Using Excel formatting is generally preferably for highly-formatted reports, and you must use Excel formatting for data sources whose application-specific colors are not supported by the Excel color palate.
The following operations cannot be used with Excel formatting:
Preserve Format from the Essbase or Ad Hoc ribbon (see Preserving Formats While Zooming)
Retain Numeric Format (see Formatting Options)