This article is from the Ballet and Modern Dance FAQ, by Tom Parsons firstname.lastname@example.org with numerous contributions by others.
Opinions vary; some say, Right away; others say, After you've had
a year or two of ballet to lay a foundation. A great deal depends on the
individual. Ballet teaches a vocabulary of movement which has largely been
rejected by modern dance; and some people find that ballet inhibits the
kind of movement favored in modern dance. But ballet is unparallelled for
strengthening your body and for teaching you to think of it as an instru-
ment of dance. For many people, the ideal may be to take ballet and modern
concurrently, if that's feasible.