Selecting data from a table without an ORDER By clause will never guarantee the order the data returns in. if you add a TOP into the mix, the data you get could be different every time you run the statement even if the underlying data hasn't changed. Having a clustered index may cause SQL SERVER to more often that not return the results in the same order, but that doesn't always mean it will. Plus, if someone (stupidly) were to drop or amend the INDEX, you could be in for a world of pain.

If you want to always have the latest 20 records returned, then add an ORDER BY clause, it will always guarantee your results will be as expected.