The lazy diver (me) puts it on and leaves it on- I never seem to need the extra stop and can get it by upping iso or a variety of other options- I like the warmer diffuse light and seem to get better results with the diffusers on at all times. By all means, make sure there is a string or secondary holding device on it though as they do come loose and fall off when you least expect it (typically at the beginning of a trip where a spare is out of the question- eg my last donation is at the bottom the the Yap trench and occurred at the start of a 2-week trip)
Thanks, Eric ... I do have little string tying it to the strobe, but it is the one that came with it and I think that's part of my problem ... it is too short and tied to the "wrong" side making it just that much harder to get it right.
I have some stretchy thin cord I use for this kind of thing in the closest and I think I'll make a new one that is a bit longer as well as a bit of tape to hold it on as it is much more squirelly than my other Sea & Sea diffusers. The YS-27 diffuser slides nicely around the face of the strobe and is very easy to take on and off ... which I seem to do a lot - and probably shouldn't! My goal is usually to find the "right" manual exposure and stick with it (f5.6-f8@160) and do most of my exposure tweaking on the strobe head [power settings.
I think I will just plan on leaving it on all the time as you do as that will also allow me to jerry-rig the optical cable to ensure/maximize proper slave firing. Not sure what I'll do with the YS-27's diffuser ... but as it comes on and off so easily, I can wing it ...