As far as mixdown goes the physical end or beginning of the project is not always what is wanted. The best practice when doing a final render is to explicitly select the region between the start and end point you want as well as all relevant tracks.
To simplify this process I add explicit markers called begin and end to the project. Then when its time to export I do this:
- open the markers view
- CTRL-A to select the entire project
- Click the begin marker in the markers view
- clrl-click the end marker (this selects only the region that you wish to render)
- export
As far as SONAR not stopping at the end, if you don't make any selection SONAR assumes the project length is up to the last event across all tracks in the project. You most likely have some event, an automation node, a MIDI event or a tempo change that is extending the project beyond the last audio visible event. There are many ways of using bounce since SONAR supports exporting specific selections so with that power comes some complexity. In a future version we'll consider improving the workflow for the more basic operations.
PS: A word of advice. Always listen from start to finish to your final mix before showing it to a client to avoid surprises like this. With any automated process there is always a chance for user error or something unexpected happening - fade's truncated, effects bypassed or something like that.