IWB only if I really have to (running shorts without pockets or pants with "thin garment floppy pockets")
In my "urban" day to day life IWB does not work at all: Shirt untucked, fleece jacket, maybe down vest, often times hip bag: Knife really not accessible.
By far the most IWB carried is my Chap FRN: Specifically bought it as a light and slim folder that still has acceptable blade length (so longer blade than a DFly for me) to be carried IWB on shorter trail runs in running shorts. Works perfectly, and before you ask: NO, no rust problems whatsoever on the HXP blade..
Sometimes I carry my Pac Salt SE or Endela SE IWB instead of the Chap when trailrunning without a running pack / running vest