Can you give a little more information about the field? At a high level, if you want a field hidden all the time you just have to be sure it is part of the record table. To check, when you select Configure > Form Layout, you are presented with a slush bucket of available fields on the left and fields in the view on the right. As long as your field is listed on the left side it does not need to be moved to the right side to be used in form operations.
The big concern with a field that's always hidden is what does it do and how does it get populated? As long as data is getting pushed to this field somehow, you can keep it hidden but still use it for reporting or any other scripted operations.
