It is good practice to tie room/space volumes to levels within your Revit model. At the outset of the project the inclusion of these should be considered and levels created to accommodate the modelling. This ensures a good clean gbXML is exported from Revit with no room/space volumes overlapping and any future updates to levels adjusts volumes automatically.
It is best practice not to use the offset function when placing rooms / spaces within Revit, except when placing roof level volumes (see below for Roof Level rooms/spaces). As mentioned above, if you are using the levels with no offset it ensures that volumes are not overlapping and are clearly defined. Place the room / space within the floor level in question, set the upper limit to the next level above and set the offset to zero.
Images from Autodesk Revit: Space placement Constraints and settings
