I wouldn't say that it matters so much. I had everything ready for my application by the end of July (letters of rec, transcripts, etc.) and then submitted it during mid to late August. Because I took the PCAT in mid September, submitting my application in August ensured that everything was ready to go and all they need was my PCAT score. I would definitely recommend doing that so that you aren't rushing to get everything in once you're done with the PCAT if you're taking it later in the year!