Terry,
You must always specify an ORDER BY to GUARANTEE that results will be returned in that order.
More often that not, if your data is physically in the order that you want, it will be returned in that order. But that's just because it's convenient for SQL Server to do so. In a multi-CPU environment they may get mixed up, plus there's a whole bunch of situations that can aslo affect the ordering.
In short, if you want an specific order, use ORDER BY.