IT depends on your situation really.
If you drive, and the cost of the fuel is affordable, and you have the time, then I think it's true to say it can be really useful to go and look at all the Universities that might be on the shortlist. Of course, those conditions don't really apply to many folk, so you have to start being a bit realistic.
I actually enjoyed doing the open days that I did with each of mine - I think at that age / stage of life it was quite a nice "day out" to spend a whole day together, just the 2 of us, or just dc and dh, but we were fortunate in that we could absorb the petrol cost and both were able to be a bit flexible with our work hours. I don't think it would be particularly helpful to do more than about 6 though - I suspect they would begin to blur after a while.
Once you've been to a couple of quite different ones, I suspect some people can then rule out others that might be on their 'long list' , having concluded they do (or don't) want to be in a big City, or they don't want to be more than 3 hours drive or whatever.
That said, one of mine went through clearing, to a City none of us had ever been to, let alone looked round the University, and she was very, very happy there and didn't really want to leave when she graduated. 