I too thought 6.7 was "optimal" but that is not case anymore with my grows and all the research I have done (soil). If you are not carefull then before you know it your soil is over 7.0 which sucks when it comes to up-take of nutes for pot. Adjust to 6.3-6.5 and you have a much better buffer zone. Once I started doing this over 5 yearts ago my plants have been much better off. BUT, to each his own.
Good luck