"I'm doing back and Biceps today, I don't need to stretch my triceps and chest"
WRONG!
antagonist muscles are used as stabalizers in our exercises and
help maintain proper form. Overly taut,
unstretched triceps, have a tendency to over stabalize your biceps, thus restricting your full potential on bicep curls. Also your pecs (esspecially upper) play a large role in stabalization of the humerus during biceps execises, especially when done with heavy weights.
tip: stretch
ALL muscles involved (primary, secondary, stabalizers etc.) before lifting.