You will definitely see problems running at a PH of 6.0. Once they start showing obvious flowers 6.0 is ok.
Pre flower I always start with a PH of 5.5 and let it drift up to 6, no higher or I see problems. Once I can see a good amount of flower production I usually let it hover around 6.0.
Ill try and find a chart that explains why.
The key to your problems was in an earlier post when you stated that the PH dropped. What you have is a deficiency not nute burn. Up your EC until your PH settles, ideally you want it rising slightly. Once it does you know your at the optimum level for that stage of growth.
Just for reference im currently scrogging 4 autos. Week 3 they were ready to go into my wilma 4 and took an EC of 1.2, week 4 they took an EC of 1.5 and they are currently in week 5 and are taking and EC of 1.8.
They are drinking about 5 litres a day and EC is dropping 0.4 in that time.
Autos are hungry!
I know your past that stage now but the info might come in handy for your next grow