Will having my ducts clean help with allegeries?

Yes! One of the most common allergies is to dust and dander. This can accumulate in your heating/cooling system and anytime your furnace turns on it puts the dust and dander into the air aggravating alergies. When you have your ducts cleaned it removes most of these particles that have settled in your system over the years. 

What should I know about those $49.99 whole house cleaning offers?

They are crap