I've done considerable research on the Ultravalve's biasing. I have built a dual bias voltage meter and have watched the behavior for some time now.
The real story is that 1) the bias is line voltage dependent, and 2) should be set after the amp has been on for several hours. I have purchased a Kill-A-Watt meter (available at Radio Shack) and that device constantly monitors incoming line voltage.
It usually takes about 3 hours for the bias to fully bottom out, so your initial setting will be a small amount off. At start up the bias usually starts out at around 1.7-1.8 volts and slowly migrates downward with time. When you finally set the bias to 1.56 volts, the line voltage should be at 120vac. As the line voltage varies, so will the bias, but should always reference 1.56 when the line voltage is at 120.
After setting the bias, you should not have to monkey with it for some time. I'd say to recheck it after 3 to 6 months.
One strange behavior to be aware of is that when you adjust one channels bias up or down, the other channel with change conversely. In other words, as you might change the left channels bias down by .1 volts, the right channel may go up by .03-.04 volts. Using 2 volt meters will quickly show the results I am describing.
I have also found some small size knobs that I have set-screwed to the bias adjustment pots, to make them "finger" adjustable. I found the knobs at a local Ace hardware store.
So basically it is set it and pretty much forget it, rechecking about every 3 to 6 months.