I personally think the drop down menu's should be onhover rather than onclick.
I was just thinking exactly the opposite. Remember there is no 'hover' on touchscreen devices so dropdown on click is what you should go for nowadays. I am looking for a good onclick dropdown for a project at the moment.
I don't think you should compromise major functionality for all the desktop audience just because of touch screen devices.
Your top level menu should link to a page with all the sub menu options inside it anyway so it wouldn't matter from a navigation point of view if the hover doesn't work on touch.
To those users they wouldn't see any difference as they could still navigate the site fine. Also its important to note that hover does actually work on iOS devices as a kind of pseudo press event - its only android that would definitely not show the hover state. Try a hover drop down menu on an iPad / iPhone - it should actually work.
I'm not sure I understand how click is compromising functionality. Click/hover, what difference?