you want to fill your grout joins "fully" (with grout) in wet areas but you do not want to encroach on the expansion perimiter joint (because that will need to be filled with Silicon)...and therfore create a watertight seal,
However.....in a dry area....( think pegion hole 3 ft above a sink)........to save time you can Silicon that prior to grouting and there is no difference......IMO.......
but everydays a school day................hehehehehehheehehhheehhehhe
Usually after, but if we finish tiling early in the day and Silicon's all there is to do we'll do it, to quote a typical dodgy builder, never had any problems