There are really two questions here:
1. Is initiative rolled once per encounter, or once per round? The text in the Initiative section of Game Guidelines implies that it is only rolled once, at the beginning of turn-based actions, but is not explicitly stated either way.
2. The Leader can use their "Battle Commander" skills as a free action immediately before initiative is rolled. The advantages of the various free actions seem to make more sense if initiative is re-rolled every round. So, if initiative is only rolled once at the beginning of turn-based actions, then how many turns do the Assault and Inspire benefits last?
Initiative is rolled at the beginning of each turn/round. The Battle Commander aspects (tactics, assault, inspire) are in effect until the turn ends.
At the beginning of the next turn, before initiative is rolled, the Leader may make a skill check and if successful choose which aspect is in effect on this turn. Thus each turn he chooses which is in effect.
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