Dare I Say It... I Dare, I Dare!
Kind of depends on the plants, but, mainly it is the critters that object to pH down in the 6.0 range, though there are critters that demand or at least prefer pH in the sub 6.0 range.
While it is true the nitrifying bacteria do not do so well under pH 6.5, it is equally true that total ammonia presents itself as primarily ammonium (virtually 100%) under pH 6.5. At reasonable level ammonium is not toxic, and is in fact, very available to plants without any help from the bugs.