There are some (relatively few) licensing situations where you'll be entitled to both the writer's and the publisher's share of performance royalties. (needle drop libraries sometimes only take the sync fee and leave you all the back end) In that event, if you're with ASCAP, you need do nothing further to get both shares; with BMI, you need to pay an additional fee (I think it's $35) and register separately as a publisher.

If your music is available on the internet in any form, you are (technically) already a published songwriter, congratulations! ;-) You could wait 'til there's a deal on the table before signing up with a PRO, but if you seriously expect it to happen there's no reason to wait, imo. Go sign up at SoundExchange while you're at it; I'm aware of a couple of cases where music was used without clearance or even the writer's knowledge, and while it doesn't necessarily pay to be prepared, it don't hurt none, neither.

up in Indiana, where the tall corn grows