There is no rule that CDs or DVDs have to be in a particular container. However, iso files cannot have more than one session. So if the disc is mixed data and audio, it can't be iso.
I'm not familiar with imgburn, but it may automatically choose iso for single session CDs, and put multi session in bin/cue. Shouldnt matter much. Most programs that read/mount images can handle both. If you really want them all bin/cue, there is probably an option somewhere in imgburn to force it.