No, not really. DOS was the basic operating system before Windows was even thought of... Yes its true, back in the old days, there was no GUI (graphical user interface) with pictures you clicked on with a mouse. It was all a black screen with a A:> or C:> prompt and you typed DOS commands to make something happen.
With all version of Windows (from 98, all the way up to 2000 and XP and probably Vista - haven't tried Vista yet), DOS is still an underlying part of the Windows Operating System. But is it necessary, absolutely not. There is an icon to go to a DOS prompt (or you can click Start, Run, command (or cmd for Win2k/XP). This lets you run some functions in a DOS environment, but most people never need to use DOS functions when running Windows. But can you disable DOS from working on a Windows Operating System such as 98, nope.... It's built into it.
So with that in mind, not sure why you asking if you need DOS for Win 98 to run correctly. It's not like theres an option in Control Panel, Add/Remove Programs, Windows Environment to disable DOS.....