My experience of keep taking in the bg's compared to the 'real' frontier, is that they work different.

My thoughts would be to kill all guards in a bg keeptake, since guards are a real threat.

In a 'real' frontier keeptake we would rush the lord only and only if the keep is unclaimed or on a very low level, ie 1-2, or if we have overwhelming troops, ie >50 attackers.

If the keep is defended and the odds are less than 3-1 the guards should usually be killed first. Even if the keep is unclaimed or at low level.

If the keep is high level, ie 5-10, the guards should be killed, else they will be wear your attack down by killing your troops.

So for bg keeptaking i would say kill the guards, else they will wear the attacking troops down too much when fighting the lord.