It depends.
My college, for instance, doesn't allow adding and re-submitting letters once they are submitted initially. So, in my case, all the letters will go to schools only once.
However, some med schools do accept additional LORs as an "update," so if you know there is another strong letter to be submitted later, you may send these as updates after you submit the initial required letters and secondaries. However, other med schools might not accept this type of updates, so you should check the websites and/or e-mail schools.
If you are waiting for additional personal letters (not science or non-science letters), and if you know for sure you can submit those letters properly (e.g., can you submit them through AMCAS/interfolio/etc. instead of e-mails that med schools might not accept as a valid method?), I think it is a good strategy to submit the required ones early and send additional ones as updates.