This is just my opinion, as this seems to vary from individual to individual and region to region. I've been adding back lane as private roads in the past as URs pop up, but I don't make them otherwise. I've seen them marked as private roads, services roads and now I see that some places are using the parking lot road type.
The problem I have with alleyways is that when Waze data for a city is in its infancy (most Canadian cities at this point) and there is little traffic/speed data and improper turn restrictions everywhere, Waze will be far more likely to route people down these side streets. Once traffic data matures and improper turn restrictions are removed, I think it'll be safe to add the bulk of these side streets in *residential* areas since speed data should maintain proper routing.
Also something to keep in mind, if you do happen to remove a back lane, PLEASE be sure to delete the junction point. This seems to be a problem in the Vacouver region and other areas that I've been trying to address. These unnecessary junction points will also wreck havok on routing. I'm not directing this message to anyone as I'm unsure who is doing it, just like to post a reminder