Both my husband and I were brought up knowing that if we eat out in public, we should be on our best behavior, and at home it is a little more lax (as it should be). Hubby and I usually eat at the coffee table in front of the tv - bad habit we cannot get out of - but I wont let Connor eat or drink anywhere but the dining room table! I'm teaching him to sit still, not move or twist around, and not to bang his hands, pull on the cloth, or spit food. Toddlers are so much fun.
I think some leniency is alright at home, why must one be formal ALL the time in front of family?
But manners are important if you are out in public, or in front of company.