Value is subjective. More often than not, we enter into the adult world with values largely taught to us by our parents and teachers.
As a parent, when you praise a child excessively, the child will over ascribe the value of that action or object. By failing to be honest about the value of child's actions, you're warping their sense of value. When you over react about an achievement, for example being shown a picture they've drawn or being shown a school report card, they believe their efforts are worth more than they would otherwise be.
To put it another way, by providing false praise (or excessive damnation) to your children, you're screwing up their value barometer.
Just provide your children with honest responses. If they show you a drawing that you don't like, let them know. If you really want to be useful, tell them why you don't like it and ideally ask them what they think. Help them discover their own metrics for assessing value and demonstrate yours.
But be aware that there can be significant, yet subtle consequences of providing false praise to your child.