If a fighter never offers an intelligent foe the chance to surrender, he should be required to make a powers check. This is not true, however, if the foe has proved to be an ongoing villain who has escaped justice numerous times due to clemency offered in the past or unless offering such poses a clear danger to the fighter’s party. As fighters advance in levels and kill more monsters and people, the DM might also call for powers checks to indicate the hardening or deadening of the character’s soul as he becomes inured to dealing out death.
For those DMs who prefer an easier way to determine when to ask for a powers check, you may assume there is a base 5% chance per level above 1st that a fighter commits some action worthy of calling for such a check. Every obviously selfless, humble and good action performed may reduce this percentage by 1%, but the chance never falls below 1% total. Those who dance on the precipice by acting in an evil manner may add to the percentage chance by +1% for each instance of uncaring or evil action. Obviously, those who commit atrocities should not receive a DM’s roll to determine if they need to make a powers check. Some things should simply spell automatic failure.