I run Grey Hunters with a Wolf Standard and I've been wondering what the best time to activate the standard is. At first I figured that you can activate it as soon as you are in combat, however that is not the case as the rules clearly state that once you activate it, the re-rolls will be in effect for the
next assault phase. With this in mind, you have to remember to activate the banner in the movement or shooting phase, either your own, or your opponents.
You can activate it in your own shooting phase, just before you charge. However with random charge distance, there is a chance that you can roll badly and then not get the charge off: Does the re-rolling of 1's include charge distance? I think it does as its part of the Assault Phase, so maybe the banner could help here. However if you do roll bad, or have a long charge that you might not make, then activating the banner at this point could waste the once off effect.
In your first assault phase, you have the charge (or counter charge) bonus attacks, so you are already buffed in the first phase. Activating the banner in the second phase, however gives you the bonus of the banner in a phase where nobody has the charge bonuses. So it gives you a second phase where you are buffed making it easier to break a unit if you didn't manage on the first turn. The downside to this however is you have less attacks, so in theory less 1's to re-roll therefore its less effective.
To conclude: When you know you will definitely reach combat with a charge, its best to activate the banner for the first assault. If you are unsure if you will make the charge, then dont risk activating the banner and it becoming useless, rather wait and use the banner in the opponents assault phase.
One last point: activating the banner in your opponents shooting phase might deter them from charging your unit if the combat could go either way; but 've yet to see this as my opponents are usually bloodthirsty