Hi Florik,
We've always played it as "+1 once per scene" no matter the type of detail. The combat was suitably fast-paced that it didn't feel like too much of a restriction (i.e., the players or the monsters were dead/fleeing after 4 to 6 rounds). If it feels oppressive, I could be convinced that "class details of +1 can be exploited once per level per scene" would work not too badly.
Best,
Slade