How important do you guys think to rank keywords when submitting to istock since checking the keywords' meanings take some time already?

Hard to know. It seems to matter if you want similars uploaded together to show as similars under the image.
However, the system changes the order on acceptance, quite considerably in 'new' view, giving generic keyword priority over specifics.
I keyword from specific to generic by natural inclination; but with the changes, I have no idea whether or not it makes any difference in search.

^^ No-one here can help you with any of that.
There is a thread on the Keyword forum over there to deal with these issues, but you'll need to be very specific and give examples (with screenshots) of what you mean to get any response. Even then, sometimes only peer opinion is given (might be insightful, might not, like here  ) or no responses at all. 

Something is seriously odd about the way keywords arrive in New View, quite different from classic view. I see keywords arriving in my files in New View which I didn't put in, they seem to be 'secret' DAs of keywords which I can't see, or else the system is guessing keywords based on those I have, sometimes very inaccurate, though as they're not in my keywords list, I can't remove them. Or something else altogether is happening.  This has been brought up from time to time on their  forum, but no-one seems interested in addressing the issue. Still, as they don't seem to care about accurate keywording any more, hell mend 'em. Last night I saw a recent acceptance of a particular Indian animal, almost certainly taken in a zoo, correctly identified in the title, but with seven other species listed in the keywords, and over 20 African locations, from all over the continent, despite it being endemic to the Indian subcontinent.

On specific locations etc.: iS used to be good for unusual locations or species, but now that it's basically a subs site, it's less worth the effort. If a photo is specifically of an unlisted location (species whatever), maybe it shouldn't be going to micro (just a thought). If it's a generic something that happens to be in the unlisted location, that might be different. E.g., a photo of McDonalds in Smalltown might sell because it's McDonalds, a photo of Smalltown's Town Hall might get you one or two subs sales, if you're lucky (or if extremely lucky, a credit sale, which is less and less likely, especially with newish files).

Adding new words/phrases to the controlled vocabularly (there's a request thread) used to take about 48 hours, now it is about eight weeks and counting.