Hhmmm. An interesting conversation.
I haven't taken any weddings since I have been digital, but did half a dozen or so with film.
Using film, I did not have the luxury of changing ISO without changing film (or bodies).
Also, not that it is relevant to the ISO debate, you are much more restricted in the number of shots you want to take when using film, so memory space should not be an issue as discussed.
Raw VS JPG - take your pick - I use RAW+JPG basic, only so I get a quick jpg to view before getting into PP.
ISO -
Generally my advice, and the way I use the camera - would be to leave it at the sharpest ISO setting (100) and adjust other aspects of the photo. Only when I need to, that is, I cannot obtain the image I am trying for with the current settings, do I adjust ISO - indoors for example, where you could soon detirmine what ISO setting would be needed to cover the shots you will be taking, then leave it set to that - and train the mind to change it again when lighting changes (eg. you go outside)
So in short, I would not use the Auto ISO setting. I like to have control of the camera at all times, not letting it decide for me.
Having said that, I said I hadn't done weddings using digital. Perhaps there is a genuine benefit to the facility - perhaps that benefit is in speed - not having to take the second or two to adjust the ISO might just mean capturing a moment that may have been missed. But I got great results with film - as did others before the digital era, and we couldn't change the ISO so why not leave it and only change it if you have to?
The feature (Auto ISO) exists, so someone thought it was a good idea, but then P
mode exists too, and I don't use that!
One last thing - I like to go to rehearsals for weddings - allows me the benefit to plan shots in my head, talk to the minister or whoever about policy for flash, where to stand, what access I have etc. AND most importantly, I get an insight to what the lighting is like - daylight from one side for example so I know what side I would prefer to be on... And a rough idea on how well lit things are likely to be.
All sorts of things.
Just read the question again before posting - you say weddings and PORTRAITS.
Most portraits I have done has been well set up - I know where I am going to be and what the lighting will be like, so I see no need at all for Auto ISO under such controlled conditions. If you need to step it down to 200 or 400 or whatever, then it will be done and away you go concentrating on other elements of your photo, no worrying about what the Auto ISO will be doing.
Just my thoughts.