This software isn't really a necessity for basic operation of your Zip or Jaz drive. But you should use it for formatting (when necessary), and it is useful for things like accessing the password-protection features, monitoring the Disk Life and Format Life status of your disks, changing the sleep time for the drive, and automatically ejecting Zip disks when shutting down Windows. It also has a few other bells and whistles which you may or may not find useful.
Since IomegaWare is not specific to Zip or Jaz drives, you won't necessarily need to re-install it for your Jaz drive if you already have it installed for a Zip drive (and vice versa); one installation will take care of all Zip/Jaz drives on the system. However, there are some drives that have a minimum version requirement. For example, an IDE Zip drive will need at least v5.3 of Tools95 (that's what IomegaWare used to be called) for the tools to work properly with it. And I think a USB Zip drive may require at least IomegaWare 1.13. As long as you don't use a version which is older than the one which came with your drive, you should be OK.
If you decide you're going to install the IomegaWare, it's best to close all unnecessary programs running in the background (everything but Explorer and Systray) first to prevent possible problems. You can press Ctrl-Alt-Del to access Win9x's Close Program window.
To uninstall Iomega's software, first close any Iomega programs that may be running in the background (i.e. Imgicon and Iowatch) using Ctrl-Alt-Del. Next, go to Add/Remove Programs in the Control Panel, and uninstall the Iomega software (its description can vary depending on what version you have installed). Now, use Explorer to verify that the Tools_95 directory (it's Program Files\Iomega with newer versions) has been deleted, and delete it manually if it's still there. Also, check for any remaining Iomega-related shortcuts in the StartUp folder and the Programs menu (the Programs menu is Windows\Start Menu\Programs, and the StartUp folder is a subdirectory of Programs). If the uninstall worked properly, there shouldn't be anything left for you to delete though.For parallel-port drives, the Iomega Parallel Port Interface should still be present in Device Manager under SCSI controllers, so your drive should still be recognized after uninstalling the Iomega software. You can remove the Iomega Parallel Port Interface manually if you like, but its presence shouldn't interfere with the installation of a newer version of Iomega's software (as long as it's reported to be working properly). However, if you'll no longer be using the drive on this computer, then you'll probably want to remove this from Device Manager.
When upgrading, it's usually best to uninstall your existing version before installing the new one.
After installing IomegaWare, you should have several new shortcuts in the StartUp folder.
Iomega Disk Icons runs imgicon.exe which has the following effects:
If you remove the Iomega Disk Icons shortcut from the StartUp folder, you should always remember to eject your Zip disk before powering down the drive (or use a 3rd-party program to replace the auto-eject function).
Iomega Startup Options runs imgstart.exe which sets some of the options on the Iomega tab under the drive's Properties in My Computer to however you last set them (I believe it reads the settings from the Registry and sends them to the drive). If you remove this from your StartUp folder, these settings (e.g. spindown time) should return to their default values next time you start the computer. This program does not stay resident in memory; it terminates after running.
IomegaWare runs commander.exe (with a /startup switch) . This is new with IomegaWare 2.0. Having this in your StartUp folder may or may not have any noticeable effect; it depends on the settings you've chosen on the Options tab in the commander.exe program (which is available via another IomegaWare shortcut on the Start/Programs menu under Iomega).
QuikSync. If you've installed QuikSync, this will be placed in your StartUp folder to load quiksync.exe with a NoStartUp option. If you haven't yet run QuikSync to set it up, then it will terminate as soon as this shortcut runs it. However, once you've set up QuikSync, it will stay resident in memory after being loaded by this shortcut (unless you then use the option in the QuikSync program to turn it off).
Iomega Backup Scheduler will appear if you've installed Iomega Backup. It runs dtiom98.exe with a /sc switch. It seems to terminate without doing anything if you haven't set up any scheduled backups. But if you have scheduled backups, then I think it will run dtsc.exe which will stay resident in memory and place its icon in the Tool Tray.
You don't need any of these. Use them or lose them; it's your choice. Just don't forget to eject your Zip disk before powering down the Zip drive if you remove Iomega Disk Icons.
I don't use these, so I haven't learned much about them. If you have questions about them, read their help files (accessible via the Applications tab of commander.exe - the IomegaWare shortcut) and the information at Iomega's website, and post any remaining questions to the alt.iomega.zip.jazz newsgroup.
This isn't always easy to determine. I'm not aware of any consistent way to tell which version you have. If you have an IomegaWare shortcut which runs commander.exe, then you have IomegaWare 2.0 - 2.2.1. Most of the information on this page applies to v2.2.1 which was the last full version of IomegaWare available for downloading. The later versions generally contain the IomegaWare version number on the More Information window in the Drive section of the Iomega tab under the drive's Properties in My Computer (or Explorer).
Some information about older versions of Iomega's software can be found on this page which I've now abandoned.