A higher pH is more critical if your pond is showing an elevated ammonia level.  Ammonia becomes more toxic at higher pH values.

A lower pH is usually a sign that your biofilter is about to or has died.  And the fish will soon follow....

Koi prefer a range of 6.8 to 8.6.  They can tolerate slightly higher and lower, but will be more subject to stress.  Daily fluctuations in pH are even a greater stressor.  The nitrifying bacterial colonies in your biofilter prefer a pH greater than 7.4.  You want your pond's pH to be between 7.4 and 8.6.  For the most accurate pH reading, you should test in the morning (the earlier, the better).

In ponds where the pH has "crashed", the pH will often rise tremendously through the course of the day (could be 5.0 at sunrise and 9.0 at sunset).  If you were to test at 10 a.m.  your pH may read 7.6 (which is a good reading) giving the APPEARANCE that everything is fine --- when in reality, it's not.  This is why the early morning reading is SO important.

If you monitor your KH and control it with baking soda, your pH will stabilize at about 8.4.