This is actually bad advice. Other than for convenience's sake, you want to RE one at a time. Otherwise you will end up with an excess of lower quality items if you happen to succeed earlier (if you craft 5 and suceed in the first try for example). The expected number of tries is the same whether you do 5 at a time or 1 at a time.
Yes, ideally you'd save a lot of mats by doing it 1 by 1. But the burnt out from the whole RE process if you do that, man....
If you're actually playing that toon, well, maybe is not a big deal stoping your questing/daily/whatever to RE one item. But if you're not, what ? You're going to relog every 5,10, 15 minutes to give it one shot at it ? Assuming that can ofc. ( Don't think your guildies will take kindly the fact your stoppin everything to RE stuff on another toon ). Getting the correct 2nd prefix crafting one item at a time is a dreadful thought tbh. And spare blue items aren't a waste. They sell everytime.
It's more than a "convenience". It's about sanity, lol