I was told, time spent fine tuning powervalve timings is worth its weight in gold.
In my opinion, this all depends on the usage.
For road racing, a finely tuned opening wouldn't net huge gains. Since once you are fully opened, you are likely to stay that way. I.E. does the Honda RS125 even have a powervalve? Or is it just chocked wide open.
MX a working powervalve is important, but there is always the gear below? I know people faff around with springs to get that powervalve open earlier/later but often it's just about shifting the timing.
I was told by a local technician at Yamaha that tuning the valves timing will net you decent gains. I was always too lazy to actually do any of this.