Maps on Android doesn't fetch data just in time, it caches ahead a little ways. You can also specifically tell it to pre-cache whatever section of map you like ahead of time.
I find it doesn't matter anyway, because the places were you find you've know data are usually the same areas that you don't need detailed instructions in - e.g., highways through rural areas. And it doesn't matter, because it will still have the instructions to get to get off on to a different highway.
I've driven all around the east (mountains of WV suck for coverage, esp. data), the midwest, and out to the west coast (Wyoming doesn't have good coverage either), and it's never once affected me.