Is it possible to create a saved search showing customers that don't have a particular subsidiary assigned to them? In criteria I have put in sub name and 'is not'. Tried many variations, but then I run the search and still get customers with the sub listed on their record. Any ideas?

not possible with SavedSearch it is possible with N/query description : select id from customer where id not in (select id from customersubsidiary)... if you don't know, how to write this sql stametment, feel free to ask... now im typing on the mobile phone

Will that ever happen? I think the Primary Subsidiary/Subsidiary field is mandatory.

multisubsidiary customer/vendor option