She didn't ask how to test it on Windows 98, she didn't ask for volunteers to test it either. She asked how many people still uses Windows 98 / ME.  She just wants to know if it's worth thr trouble or not.
Personnally, Donna, I wouldn't bother. But it depends what's your goal when making a skin. If you're doing it for the users and want to have it work for almost everybody, then yes it's worth it.  I make skins primarely for myself, then since it's done anyway, I upload it for others if they want it.  If it works for them great, if it doesn't sorry.  But that's just me, I have no pretention of being anywhere near a professional skinner, I skin for myself.