Jenna had all hers at the same time. I actually was OK with doing it this way - I think the longer you drag them out, the more they remember if that makes sense?
Jenna really did know about her 18m needles - there is a consciousness about what is happening that wasn't there earlier. Luckily its a couple of years before they need anymore, so she can get over it all and have fun trips to the dr again before needing another needle.

I'd personally be a little worried about splitting them, and dragging out the inevitable? Specially if you have a sensative soul. They might remember from 15m to 18m, and be a right bugger at the next set.

I guess you have to weigh up the good with the bad??