7.5-8.0 dKh is where I keep mine. If you carbon dose at all (biopellets, vodka, etc) you want it below 8.0 dKh to prevent alk burn on the tips of acros.
Some believe you will get better growth with higher alk numbers so they chose to run higher. Some believe you get better coloration at lower alk numbers, so they do that.
Natural seawater is about 7.0 dKh. I've received acros direct before and the alk is usually at 7.2 dKh, though there may be some bacterial interference while being shipped.
The most important thing is just a stable alk level, as long as it is within accepted ranges of 6.5-11 dKh.