I wrote a long essay for this thread giving my experience. My browser crashed and it disappeared into nothingness. I don't have it in me to re type the whole thing.
This discussion appears to be all B/W related. I will throw my 2p in about colour.
Unlike b/w colour can stand over exposure. When I do multi-exposure shots of varying subject I never expose under 2/3 of stop (anything less produces insignificant noise) and try not to exceed a cumulative exposure of 0 1 stop over. I am careful about my set up and film. Fuji colour print film does not suffer over exposure as well as Kodak's portra film. When setting up if one of the exposures is to have greater significance I give it more exposure at the expense of others.
I tend to add time for each exposure as follows:
1st 2/3 under
2nd 1/2 under
3rd 1/3 under
4th on the mark or 1/4 under
Examples (please excuse the crap cc and the fact one of the images is flopped. I did these scans in a rush the night before a show and have never returned to fix them.):