Hi, thanks for posting!
It's the intention of NCrunch to avoid manipulating the VS/MSBuild build process for the foreground solution, as this is very difficult to do reliably. Clearing out NCrunch data files using a 'Clean' step in the build would involve making changes to either your project files or your global build targets on the workstation. The risks of these sorts of changes (and the effort involved) would likely outweigh the benefits from such a change.
The contents of the NCrunch cache files can often be considered quite important, as this data can only be reproduced through a full end-to-end test run using NCrunch (which can take hours on some solutions). Is there a reason you want to routinely clear this data out?
Cheers,
Remco