Let me try to explain why you should not do that, why you should never assume that an SQL-product will return a result set in a specific order, unless you specify so, whatever indices - clustered or non-clustered, B-trees or R-Trees or k-d-trees or fractal-trees or whatever other exotic indices a DBMS is using.

Your original query tells to the DBMS to search the SensorValues table, find rows that match the 3 conditions, order those rows by Date descending, keep only the first row from those and - finally - select and return only the SensorValue column.

SELECT TOP 1 SensorValue
  FROM SensorValues
  WHERE SensorId = 53
    AND DeviceId = 3819
    AND Date < 1339225010
  ORDER BY Date DESC ;

These are very specific orders you have given to the DBMS and the result will most probably be the same every time you run the query (there is a chance it might not, if you have more than one row that match the conditions and have the same max Date but different SensorValue but lets assume for the rest of the conversation that no such rows exist in your table).

Does the DBMS have to do this, to run this query, the exact way I describe it above? No, of course not and you know that. It may not read the table but read from an index. Or it may use two indexes if it thinks it's better (faster). Or three. Or it may use a cached result.

What is guaranteed though is that it will return the exact same result, every time you run it - as long as no rows are inserted, deleted or updated.

Now lets see what your suggestion says:

SELECT TOP 1 SensorValue
  FROM SensorValues
  WHERE SensorId = 53
    AND DeviceId = 3819
    AND Date < 1339225010 ;

This query tells to the DBMS to search the SensorValues table, find rows that match the 3 conditions, order those rows by Date descending,, don't care about the order, keep only one row and - finally - select and return only the SensorValue column.

So, it basically tells the same as the first one, except that it tells that you want one result only that matches the conditions and you don't care which one.

Now, can we assume that it will give always the same result because of the clustered index?
- If it does use this clustered index every time, yes.

But will it use it?
- No.

Why not?
- Beacuse it can. The query optimizer is free to choose a path of execution every time it runs a statement. Whatever path it sees fit at that time for that statement.

But isn't using the clustered index the best/fastest way to get results?
- No, not always. It most probably is the first time you run the query. The second time, it may use the cached result. The 1000th time the result may have been removed from the cache and another result may exist there. Say, you had executed this query just before:

SELECT TOP 1 SensorValue
  FROM SensorValues
  WHERE SensorId = 53
    AND DeviceId = 3819
    AND Date < 1339225010
  ORDER BY Date ASC ;         --- Notice the `ASC` here

• And the cached result (from the above query) is another, different one that still matches your conditions but is not the first in your (wanted) ordering. And you have told the DBMS not to care about the order.

OK, so only cache can affect this?
- No, probably many other things, too. Like other indexes that could be considered (at that time) by the DBMS as better for this query. Like if some future developer changes or completely removes this clustered index you have. Like if you or some other developer adds another clustered index (SQL-Server does not have such feature now but it may have in the future or you move to another DBMS). Like if you updated and the new optimizer has a minor bug.

I hope you are convinced that you should never rely that an SQL query will return results in a specific order, unless you specify so. Amd never use TOP without ORDER BY, unless of course you just want one result and you don't care which one is returned.