Your iPhoto library is as safe as any folder with photos on your Mac.
But you have to keep backups of your photos, just like of any other documents, because
- someday you might make a bad mistake and accidentally delete the wrong photos - nobody is perfect,
- or you might install malware and corrupt the system,
- or you system drive or the processor of your mac will fail, and that one day the computer will fail is a certainty. The only question is, when it will happen.
So it is essential, that you have a good backup of all files on your computer, for example a Time Machine backup, and keep a copy of your iPhoto library on a second drive. If you only keep copies of the photos you do not have a copy of the edits and titles or keywords you added, so you just could recover the photos, but not the work you invested.