I'm still waiting on my committee letter but I submitted all secondaries (that were made available). Certainly won't hurt if you get them done now. Some schools hold off on sending their secondaries until your application is complete (with scores and letters).
The schools that require you to just pay, I would wait until the LOR is submitted and received so you don't waste $$$ if the letter doesn't get through.
I would still work on the supplementals that require essays while you wait - some of them are really long.
@redhotchiligochu Do you know where the list of schools that require essays is?
And thats what I figured, I don't want to get my application thrown out just because it isn't completed upon the first time being evaluated.