This is exactly how it already works. Tiles are recalculated/cached when they have not been updated (on your client) in the last week or so, when you route through them or when you manually refresh them. But (re)routing and point collecting, at least on the iPhone, are online functions, I wouldn't use it offline and consider flatfee internet on your phone. Though the telco's are limiting it more and more, you can still get unlimited data in The Netherlands for as little as € 10 or less. Are you not that lucky?

Yes, that's true. Tiles are 'zoom-level-specific'. But I tested this just last week and disabled mobile data. On one zoomlevel I watched (by swiping the clientmap along the purpleline) the entire route at home with wifi enabled and let Waze download/cache all tiles on that zoomlevel. Than, while driving the route with dataconnection disabled, Waze showed all tiles on the 100km route, as if data was available. You are right that when I zoomed in or out, it showed blank tiles. So I guess indeed I was not specific enough in my answer, thanks for pointing that out.

@Brutus: It is my impression that, the way you describe it, is NOT the way it works. Tiles are pre-loaded - but not all of them in the highest resolution. The highest resolution-tiles (the ones where you can recognize streetnames) seem to be downloaded only on the fly. You can try this by using a slow internet-connection: first, clean your cache, then generate a long enough route. Next, zoom deeply into that route, somewhere in the middle; as you progress over the blue line you may notice the delays where Waze loads the missing tiles, you may also see the progress of the data-counter. If you would have driven off at once after generating the route, you would get stranded in the "desert" at some point - unless you zoom out far enough to see the roads in lesser resolution.