Not being an expert on the topic but also just having installed one on my kettle; I placed it very near the top of the kettle so that the discharge opening would be an inch or so below the average wort level. I use mine to recirculate while chilling with the IC, but will be switching to a plate chiller next brew (still setting-up the hoses and connectors).
I've seen recirc fittings located near the bottom and I believe this location would be good for whirlpooling. I noticed I did get a decent whirlpool with my discharge near the top of the wort, but I have to imagine it may be better at the bottom.
If you're planning on recirculating only for chilling, I'd say put it near the top. Some put the fitting in the lid - either way should do the trick.