Much of this is just personal preference. While some women prefer to always wear their shirts untucked, others can't imagine not tucking them in. Some professional environments require all employees to tuck in their shirts, so make sure to read all dress requirements.
As a general rule, shirts with long tails should always be tucked in, especially in professional environments. If you hate tucking in shirts, but your shirt's rather long, add a belt to it and wear fitted pants or a skirt. This gives you a dressier look.