I will never recommend a retention strap on an IWB holster. Your belt will cause enough friction resistance to keep the gun in place. The other purpose of a retention strap is to prevent gun-grabs. If you are carrying IWB, that typically means concealed so a gun grab is extremely unlikely (more unlikely than you needing to draw). If you have to draw, a retention strap will just slow you down.
A person trying to grab your gun doesn't really care if it is "extremely unlikely" that he does so or not. Whereas many (if not most) of my iwb rigs rely on "friction resistance" for retention purposes, I see nothing wrong with relying on a retention strap to help prevent bad people from grabbing a gun from good people. In addition to gun retention training that had nothing to do with holster configuration, my agency required us to use a holster with a thumb-break design, even if the gun was being carried in an iwb rig. Good training and lots of practice permits a fast draw, even if the pistol is being retained by a retention strap-and, at the same time, helps thwart a gun grabber from gaining access to your gun.