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Take a look at this interesting article, here on ITKE:
Paranoid About Hard Drive Security? Try This
If precautions aren't taken, file fragments and even some whole files can usually be recovered. Usually, the larger the file, the less likelihood that anything more than fragments can be reconstructed. Whether it's worth any effort or not can only be determined by you.
If you're storing actually valuable business secrets that some nefarious persons are seeking, then it's probably worth some extra effort. What is the likelihood that some industrial spy is going to track and obtain the system for analysis? Or is there legally restricted private info about customers that needs to be protected?
There is the theoretical possibility of data recovery. And there is also the practical view of some uninterested 'lady' applying recovery techniques in order to extract data bits.
At the first level, simply deleting files might be enough. Much of the data bytes could still be recovered, with less and less being recoverable as the PC is used in the future. The data is not recoverable without using some software utility that is designed to do it. The ability to recognize the meanings of recovered data also decreases as usage continues.
But if no attempt at recovery is done, nothing will be recovered. If you bury something in your back yard, it's not going to be found unless someone actually starts digging around. By simply looking at the yard, you can't tell if something is underground. In the case of deleted files on disk, there isn't even a visible scar like what might be left after refilling a hole.
We can't really judge "safety" in your case without knowing what might need to be protected.