ALL cases.
All of them. Statistics are what the optimizer uses to determine how many rows are likely to be returned from a given table or index. Without statistics, the optimizer will always scan and perform hash joins. You'll likely never see a seek or other kinds of joins where they would be appropriate based on the data & the query.
And it's not a question of YOU using statistics. The optimizer uses statistics. You need do nothing.
EXCEPT
You need to ensure that the automatic creation and update of statistics is enabled (in the vast majority of cases, there are exceptions where you turn these off, but they are exceedingly rare exceptions). Next, identify places where the automatic maintenance is not adequate and provide manual maintenance.
You simply have to ensure that statistics are there for the optimzer and that they are as accurate as possible. You don't need to interfere in any other way to ensure that statistics are used.
For scads more detail, I'd suggest getting a copy of my books linked below.