It is entirely a matter of preference, but if I am reconning multiple vials, I will recon the first, then extract that and recon the next vial with the stuff from the first, etc. I think Jeff made a video on this a month or two ago.
The main reason I prefer it this way is that one time I was reconning 10 vials into one cart and only using .3 ml per vial was not going to dissolve them and the loss would be excessive.
Things can go wrong either way, but I prefer this method.