The term "sanitizing" is not used to reference this, as commented on, what you are describing is "validating" the user input. That should, of course, happen. Many validations will result in only accepting input that happens to be safe for many uses - i.e., if it's a valid number between 1-100 you could of course send it to an integer field in a database without doing any special encoding, but I wouldn't rely on my input validation doing this in my model layer.
Encoding a "safe" value doesn't make things any less safe. Failure to encode it, however, leaves potential holes in your application. Something may bypass input validation and be given to the database as an unsafe, unvalidated value. Usage of the value may change (new functionality using it differently, changed storage in database, etc) and in the new usage the value may not be safe.
Input validation is obviously something you want to do, but it should never be relied upon for protecting from injection attacks.
Encoding a "safe" value doesn't make things any less safe. Failure to encode it, however, leaves potential holes in your application. Something may bypass input validation and be given to the database as an unsafe, unvalidated value. Usage of the value may change (new functionality using it differently, changed storage in database, etc) and in the new usage the value may not be safe.
Input validation is obviously something you want to do, but it should never be relied upon for protecting from injection attacks.