you could...it shouldn't be started on seedlings. after they get three sets of leaves, not including the cotyledon, is a fine time to start.
as long as the stem is not to stiff, you can do it anytime. although, if you just need to do a branch or two, supercropping is the way to go.
search 'supercropping' on YouTube. there are some pretty kool vids there...
I got 25 plants growin, about 75% of them are LST'd. check it out.