It depends on the iOS app being used - some will download the maps on to your phone so all you're using is the GPS and a small about of data for real time traffic to re-route you - again, assuming your app supports that. I know that my old Navigon app (now obsolete) used to download all the maps over wifi and not use much data - but Google Maps (and maybe Waze, not sure) seem to download more as you drive along.
If anyone were so inclined, going to the Mobile Data section in Settings of iOS should allow disabling Mobile Data on a per app basis.
Like anything though - if something works well for your needs, then it's the best option for you