Never on the 6 but I tried it on my 5 and in my opinion if your going to get your phone wewt I would do it. It puts to much pressure on the rubber gasket around the inside of the case.
There is a company called Teqii that sells glass screen protectors made to fit the Nuud series case. They should be releasing the one for the 6 in about a week. They are available for preorder right now.