Generally Wifi is prioritized. I don't know if it's fixed, but there's a few rare things, like visual voicemail that only can use cellular data.
There's situation where will will disconnect or be automatically turned off, that tends to be a bigger issue.
If you want to keep better tracking, consider using the app "My Data Manager" (free), and it will show you a graph of recent usage, with different colorations of wifi vs cellular.
-Alex