We got our Ergo, with insert, just at the edge of the recommended range for the insert -- I think DD was 13 weeks. We did use the insert a few times, but by 15 weeks we were just doing a straight front carry.
The insert is pretty confusing; it's hard to imagine using it for a baby who doesn't _need_ it. I'd probably recommend not bothering -- you wouldn't use it for long, and there are workarounds. When it's winter (well, okay, soon it won't be! perhaps where you are it already is over!) a puffy snowsuit can play the same role in letting you carry the baby in a "sling" position. Or, as long as your baby has good head control (DD did quite early on) you can let him/her keep one or both feet tucked in at the bottom, instead of splayed out on either side of you... that was definitely more comfortable for DD at first.
Academic mama to Miss T. (09/05) and Miss V. (05/09).
Oh, and (hence )