If you want to double-check whether you are indeed streaming a song or not, go into the iTunes app while the song is playing in the background. Click on More... then Downloads. It will tell you if anything is being played/streamed from the iTunes server.
Personally, I have not had an issue with songs I have already downloaded being re-streamed from the server. This usually means the iTunes Match database on your device has been corrupted. You can fix it by rebuilding the iTunes Match database (basically, it involves turning iTunes Match OFF, wait a 5-10min, turn it back ON, wait another 5-10min for the database to recompile, and you're done.
Turning off the "Use Cellular Data" sliders *seems* like it would help, but in a very importnat case, it doesn't:
- Fire up your iPhone or iPad and start downlaoding a big playlist or artist grouping on WiFi with the "Use Cellular Data" setting turned on.
- Leave the WiFi network.
- Notice that you are still downloading and turn off the "Use Cellular Data" sliders.
- Notice a few hours later that your battery is almost dead and that you've still been downloading.
- Get an angry text message from ATT about exceeding the limit of thier 'unlimited' plan.
- Enjoy 2G EDGE data speeds for the rest of your billing period.
There are serious bugs with iTunes Match and iOS devices that Apple really needs to fix.