You should be taking your temperature that way normally.
Just sticking your thermometer in your mouth over your tongue, or slipping it just under the front of the tongue, isn't accurate. You should put it under your tongue and then push it back as far as it can go. You'll feel a little pressure and that's how you know it's in the right place.
So, it tells you to do it that way because that's how you get an accurate temp reading.