If you're doing extract, you really don't need to worry about it too much. Whatever BeerSmith predicts should be accurate as long as you added the right amount of extract and your final batch size is correct.
FG is more important for extract brewing, as it tells you when fermentation is complete. And fermentation naturally mixes everything, so there shouldn't be any issues with your FG reading.
You can't be a real country unless you have a beer and an airline. It helps if you have some kind of a football team, or some nuclear weapons, but at the very least you need a beer. -Frank Zappa