Yes, I believe it is OK, and I do let people know what I want. Additionally, I prefer surprise gifts. I know these answers seem at odds with one another, but you can give general ideas (hints) as well.
I really enjoy loose leaf teas, so I might let my wife know that I would really enjoy new teas for Christmas or my birthday.
Often, she'll deviate and get me something else, and she is really wonderful about remembering things I need that I don't even remember, LOL!
I don't want anyone who is willing to spend their hard-earned money on me to waste that hard-earned money.
So yes, not only do I believe it is is OK to tell someone what you would like as a gift, I believe it is important to. They can still deviate and purchase what they want if they care to.
I understand the modest "you didn't have to get me anything" idea, and the thought certainly does count, but re-gifting has become really popular for the exact reason of not letting people know what you want.
Ironically, re-gifting then perpetuates the problem. Excellent question :-)