That's really up to personal preference/opinion/experience.
Science says higher absorption rates at a pH closer to the higher side of the scale. But I would recommend you do your damnedest to just keep it within the 5.5-6.5 parameters at all.
If you're going to be doing DWC in a single 5gal bucket, the 3-4 gallons of water it can hold without drowning the netpot, will be a constant, DAILY if not hourly, battle for you to keep your pH stable at all. And let's not even start on keeping your waters temperature under control.
If you've got any option to upsize your container to some kind of BLACK "tote" of a LARGER volume, I can't tell you how much easier it will make your run go for you. Just cut out a hole in the lid that will fit your netpot. Even if it's one of the models built into a 5gal bucket lid.