Teens are master negotiators and manipulators. They're adept at spotting any sign of parental weakness. When you waffle and give in to their pleas for leniency, they're going to expect the same response every time they misbehave or break a rule.
Being consistent about teen discipline also means that both parents need to be on the same page. If one parent always says "yes" and the other always says "no," your teen is going to know exactly which parent to ask.
While you're being firm, don't forget to also be fair and understanding. A little empathy goes a long way when disciplining teens.