Assigning it as a page property would be helpful for concepting. I just ran into this myself as I was trying to do multiple layouts with varying document and baseline grids. They should make ALL of the grids pertain to the document at the very least, since they are document specific. There is no reason we should be required to edit the app preferences when a document had a different baseline or document grid setting.
Many of the documents I work on come from other countries where the page sizes are different. When A document had proper baseline grid settings for A4 using metrics for example, moving the layout to an American format with inches, picas, and points can have a negative effect on the rendering output of the page. Also if I change my baseline grid settings to match what someone in Germany had done, now I need to remember to change them back for American documents. These preferences should at the very least travel with the document and NOT the application preferences. A default maybe, but we should be able to change them per layout.
