There is a lot of tapping of trees in various places in the world that has gone on for many years with out any obvious major impact, and in some locations like Canada the same trees are tapped year on year without any real detriment to the trees, when I was doing it here in the UK I was turning my own hard wood taps and bungs and thought I was doing a carefull job of it, I don't know if it is due to climate or strains of fungi that we have here, it does appear that trees in the UK are more susceptible, I suspect that it maybe due to our warmer maritime climate that most silver birch here have a lot thinner bark except for those growing at higher colder altitudes.
I had a look on youtube for a good example of the method I now prefer and didn't find one that really covers it, this was the only one that comes close unfortunatley it isn't the best of vids, if the guy on the vid is doing your head in fast forward to 20:32 on the vid the last bit explains a bit about injury to the tree.
https://www.youtube.com/watch?v=H2CmIEQF6kU