I think it should be fine to use a bi-directional roadway (i.e. the undivided sections of roads) as a way in a boundary relation, but in a case where the roadway splits into two one-way lines (i.e. divided by a median), I'd roughly trace a new boundary in the middle—not the median exactly, but just between the two ways—and avoid snapping that boundary to any intersecting roads or median breaks. I would also avoid snapping any landuse or natural feature multipolygons to these roads because it's convoluted enough having a way serve as both a routable highway and a boundary of an entity, not accounting for any transit or a road route relations the way is a member of.