NSES Content Standard E 
Science and Technology: Abilities of technological design 
Grades K-4, page 137 

Evaluate a product or design. Students should evaluate their own results or solutions to problems, as well as those of other children, by considering how well a product or design met the challenge to solve a problem. When possible, students should use measurements and include constraints and other criteria in their evaluations. Students should modify designs based on the results of evaluations. 

Benchmark 3B The Nature of Technology: Design and Systems
Grades 3-5, page 49
There is no perfect design. Designs that are best in one respect (safety or ease of use, for example) may be inferior in other ways (cost or appearance). Usually some features must be sacrificed to get others. How such trade-offs are received depends upon which features are emphasized and which are down-played.