Pre-mixed juices are made in large batches, then bottled and sit on a shelf in a warehouse waiting for shipment; therefore these juices don't require steeping.
Custom-mixed juices don't get made until you make your order. These vendors give you options on pg/vg ratio, flavor bumps, added menthol, etc. Depending upon the flavoring agents, these may or may not require some steeping for 1 - 2 weeks after arrival to your home. Complex flavors such as bakery, coffee, and tobacco blends often require the longest steep times for best flavor. 100% vg juices will also need additional steep time as it is not as good of a flavor carrier as pg is. However, once steeped its flavors are as good as a pg/vg ratio IMHO.
I take the bottle cap off overnight when I receive them, recap the next day, and just allow them to sit in a dark cool place for at least a week, shaking the bottle daily if I think about it.