Personally, I am going to send them after I receive my secondaries. There's no point in sending them if I am rejected before the secondary; also, the letters might just get lost somewhere if they don't have a file to put them in yet.
P.S. I had to Google how to spell it "receive" after I saw how you spelled it. Derpface.
If your stats are high enough that you won't get pre-screened, there is no harm in sending them now.
If you're concerned, then wait until you receive a secondary. This isn't a big deal and is in no way going to alter an acceptance vs rejection decision.