I have always used NULL values, although I have seen some (heated) arguments over the years against it!
Unfortunately nobody has been able to convince me yet 😉
There are more than enough functions available in SQL to handle nulls - NULLIF, ISNULL, COALESCE, IS NULL, IS NOT NULL etc.

I agree with some of the other comments that null values should indicate 'missing' and/or incomplete data.
For example: Let's assume you want to log the start and end time of an event. The start time might be available when the record is inserted, but not the end time.
So a quick query for null values in the end time column will show you events that are still waiting to be completed.

PS. Reporting Services also support null values which can be used to your advantage.