If I had a quid for every time I've heard that 'buffer' crap, no offence intended mate.
It should be around 6.5 and 6.8 being about max in soil but then again it's all strain/pheno dependent, 6.0-6.3 is far too low regardless of strain though.
PH changes through the grow because PH is related to Nitrogen uptake.
You can take the easy/lazy route and don't bother to PH or EC sure, a lot of folk do, but your plants won't be anywhere near what they could be given ideal conditions or as close to ideal as you can get.
PH directly affects nutrient uptake.