It's wherever you want them. Maybe check out some other shops to get a feel for how much space you want from bench to bottom of cab? Make it higher, rather than lower, so you can have the space to work....also to install light fixtures if you plan to do so?
I'd make them high enough to be out of the way but easily within reach for the lower shelves. If I need a step to stand on the top shelf, that's fine, usually, that's were you put the odd stuff you don't use all the time.