if they are functioning correctly. I haven't seen/heard of any acclaimed posts to point to the benefits of those "Guaranteed To Pass" type emission products. I try to do all of the basics - Engine is running flawlessly wrt to idle and settings. Fresh oil change. Recently cleaned air filter(s). Sometimes a higher octance gas or octane booster. Fresh set of NGK plugs gapped to .035". Immediately prior to emissions test, do a minimum 20 minute sustained high speed drive and don't let it idle long prior to test.
I have however heard of successfull tests being accomplished when people have disconnected the #3 and #6 injectors just prior to driving inside the test facility. There's also the disconnecting of a vaccum line from the Dutch TT site - La-Z-Link but there's been conflicting reports as to it's effectiveness.
