I'm "cleaning" the MacPro i use at work (editing) and everytime i do disk repair (with disk utility) to the internal hard drive, i get loads of error messages. at the end of the text says "permission repair completed", BUT i run disk utility again and the errors appear as before. I also used the onyX application but everything stays the same.
SO, since i'm doing cleaning and backups these days, should i format the entire internal HD and begin from scratch? Is that really useful on a Mac?
If the only errors found are on permissions, there is no need to reformat. Mac OS has had that behaviour for years, where the few same permissions are always seen as having the wrong settings. As long as Disk Utility finds no fault in your Catalog, Multi-Linked Files or Ressource Forks without being able to repair them, don't go overboard. If DU does find such errors and can't fix them, DiskWarrior is the step you should take before formatting.
Mac OS doesn't need to be reinstalled very often. As long as you regularly empty caches, run maintenance scripts and optimize the drive, the OS's footprint doesn't bloat too much. Most reinstalls or retrieves I've had to do in the last few years were caused by hardware failures, not software issues.