No lifting of the fingers of the upper hand is required. I find it very difficult to argue with Alex's logic. Just before moving on I would like to share some findings from my teaching with you. In my classes with beginners we have a little chat and then look at a golf club before moving on to holding this wonderful implement. Having set the upper hand correctly I often take my students through BOTH overlap and interlock grips.
New golfers find both grips really strange.
These strange feelings are quite natural and, if you are a beginner, don't panic. In nine cases out of every ten I have found that beginners opt for the interlocking grip. When I ask them why they made this choice they invariably cite the first knuckle of the forefinger of their upper hand. They tell me it presses into the palm of their lower hand just between the little and third finger. If we are honest it is the feelings of strength, stability and comfort that makes us opt for, and stay with, one grip as opposed to any other.