Personally I look at it like this. This is my opinion based on our experience from customer experiences plus our own testing.

Customers should be using the latest version of the browser that suits thier operating system.
In A Nutshell this is what I recommend.
On Windows 95 I put Internet Explorer 5.01 Service pack 2 (Nothing else)

We have tested Internet Explorer 5.5 on Windows 95 and have found problems with either the installation or with being able to successfully uninstall it.

On windows 98 I put Internet Explorer 5.01 Service pack 2 (in my opinion is the best)

We have found from our own testing that Internet Explorer 5.5 on Windows 98 has some strange behaviours. While it does work, it does have some behaviours that lead us to not recommend it on Windows 98.

We have also found that Internet explorer 6 causes some of the functionality of other windows programs to break in some cases. We have also experienced problems with system shutdown after Internet Explorer 6 has been installed. There may also be some issues with uninstalling IE6 on Windows 98 systems.

Basically we conclude that Internet Explorers 4 and 5.01 were designed around Windows 95 and 98 and we recommend that customers use 5.01 SP2 on Windows 95/8

On windows ME I put Internet Explorer 5.5 Service pack 2 and patches

Personally I would only use Internet explorer 5.5 on Windows ME machines. From our analysis it appears to have been designed around Windows ME. The important thing to remember is to update the security patches. The security patches close any security holes in the Internet explorer web browser that allow unauthorised persons or viruses from exploiting security holes in the software in order to run malicious code or programs on your computer.

On Windows XP I put Internet Explorer 6 and many patches.(The patches are VERY important)

All the comments above in relation to Windows ME apply here as well.
This part is important: If you have a Windows XP machine then I would recommend you update the security patches for it. There are a number of security as well as stablity patches for Windows XP. These should all be installed.

If you are purchasing a new computer with XP installed, then DEMAND that the patches are installed before purchasing the machine. If the supplier doesn't understand what you are talking about then I would suggest choosing an alternate supplier, as they will most likely be unable to help support you, past selling you a computer.