A tablix is going to display 1 row for each row in the dataset unless you create a row group.

That's all there is to it.

Your probably getting some cool display like

20092010201120122013

1

1

1

1

1

11111

You can cheat and hide the detail row and Simply just display the total row.

But usually in this scenario there would be some requirement to group the "Sales by Year" etc.

Your assumption to perform the aggregates in the dataset is correct. the more rows you send to the report server the slower the rendering will be.