This is your decision and depends on when you have complete scientific work ready to share. In many cases, where the manuscript will be sent to journals as well, preprints are submitted close to the time of journal submission. Several journals have enabled one-click manuscript transfer from bioRxiv. Some scientists may want to post a preprint publicly for a few weeks before submission and point their community to this version for initial feedback. Some editors invite submissions from preprint servers, and some journals have even appointed specialized “preprint editors” to do this work.
Some investigators may decide to submit after receiving the first set of reviews. However, it is important to note that some journals do not allow new versions to be submitted after peer review for continued consideration at the journal.
It’s important to consider the policy of the preprint server as well. For example, bioRxiv allows submission anytime prior to journal acceptance, but not after. If the version posted to another server is the one that was accepted by a journal (equivalent to the AAM, or “author accepted manuscript”), this is considered a postprint instead of a preprint. You can check journal policies about self-archiving (also known as Green Open Access) at journal websites and SHERPA/RoMEO.
Whatever you decide, it’s best to be transparent with the journal editor about your preprint by notifying them when you post or in your cover letter.