You can never over research a destination - particularly old-world Euro destinations.
There are so many hidden gems that unless you have unlimited time and resources you just won't find them.
A tropical paradise may well need little or no research but there is so much you can miss in those "old" places.
Having said that, you definitely don't want to lock in an hour by hour, highlight by highlight itinerary that doesn't allow some spontaneous sidetracks!
I disagree with the no internet idea if you're travelling with kids or a fairly loose itinerary. Life is so much easier when you can make last minute sidetrack or train bookings online.
I've done the backpacker early and mid 90's couples trips around the world for months with little planning and no mobile or internet and then a couple of years ago a family of 4 Euro trip with the mobile devices and it is much easier and more interesting to make last minute side trips that people recommend during the trip with modern tech.
Wired is the only way to go - especially European... IMO