From your description, you probably don't need to do a low level format. Still, here is my general answer to the low level format question. Other people may give different advice.
Magnetic codes on disk fade over a period of years. If nothing were done about it, a drive's data could become unusable after say, 10+ years. People who keep long-term archives on magnetic media (like NASA & various government records offices) need to copy the archive tapes on a regular schedule every few years.
Copying data off the drive and then back refreshes the data sections, but does nothing to the small pieces of format code that track and delineate the data sections. Low level formatting wipes the disk and puts down all new format code.
Low level formatting is thus used to refresh disks that are a few years old (say 3+) or that have had problems. Downside: It can take a while with large modern drives. If you have an older interface like narrow SCSI or IDE-16 combined with a really large modern drive, you could be waiting a few hours for the low level format to finish.
If the drive is "factory new", you don't need to unless your Mac complains about the disk after normal formatting. The factory should have already formatted it. You only need a normal format to install the Mac drivers, partition map, volume bitmat, and root directory.
If the drive is "new to you" it would be a good idea. Never know what might have happened to it in the past.
If it is your drive and is 3+ years old, go ahead. This is cheap insurance for future drive integrity. <back up first>
If it is your drive and has had problems like lots of crashes/lots of need of drive repair utilities, again go ahead. This could solve a lot of troublesome behavior in the future. <back up first>
If it is your drive and you have to recover lost data from it, do not low level format or zero blocks until you have all the data off it, or give up on getting any remaining data off it. Both low level formatting and zeroing will wipe any trace of your data away.
If the drive in question is an Iomega Jazz drive, search the Peripherals Forum for Jazz info. From the sound of it, low level formats of Jazz disks are risky and may cost you the disk. You might have to return the disk to Iomega for a replacement.
If you do decide that you need a low level format based on the above reasons and the drive in question is ATA/IDE/EIDE, use the "Zero All Blocks" option instead. ATA-type drives don't allow a true low level format. Zeroing is not as good, but is the best option you have available. Recent versions of Drive Setup dim the Low Level Format checkbox when the drive is an ATA-type.
Finally, if you have the time to burn and no data to recover, just do the low level format (or Zeroing for ATA) anyway. You might not need it, but it is cheap trouble insurance. Even the factory can make mistakes sometimes.
Hope this helps.
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