I only transfer when client arrives.  Why do you want to transfer everybody at once?  This would solve that problem.

I transfer as they come in also.

I'm another one that transfers as they come in

Me too.

If you decide to transfer it all at once, an unstarted tax return (therefore with errors) would not be able to to be e-filed, so you would not e-file that in error.

I always leave the efile PIN empty until the return is complete and ready to be filed, then I convert for efiling in a separate step from actually filing....I wait for the end of the day and efile all the days clients that have been converted for e-filing.

I leave the e-file PIN/date blank until I have finalized the tax return too.

Interesting how you convert it in a separate step, and file them all at once.  What is the purpose of that?

To keep from efiling a return in error, you should check the EF option "Only Transmit...when Return Ready...." that way you need to make that selection per return.  And as others have pointed out, the disadvantage to transferring all clients at once might add to confusion for tracking live ones.

I convert and file separately due to CA.  
If I convert both Fed and CA and transmit at the same time, if there's a pending update for Fed it wont convert or transmit Fed until I update BUT, CA will still convert and transmit...if that update happens to change the Fed figures Ive got no way to get the CA return back and it may have gone in with incorrect Fed information.

Hmmm.  Good idea.  That same thing happens to me for Minnesota.

Yeah, I started converting and transmitting separately this year.  I don't even remember exactly what the issue was, but I had a federal reject but the state went thru and then we had to amend the state once we got the federal correct.  It was enough of an administrative mess that I'm now willing to invest the extra 30 seconds in "convert" then "transmit" vs. "convert and transmit".  Most of my ProSeries returns are multi-state so I also want to avoid the "OSC" domino effect.

Rick