I see a lot players here saying that they are following the opener guides and still failing at the opening game. I was one of these guys and then I realized I never dropped the SM banner or always forgot about it. For the opener it is an absolute game changer. An extra stun that you can add on top ASM and Scouts CC and it has a shield. It will always force the enemy to run out of it's range, so those opening point captures that you are struggling to capture become easier to take. Mid game it is great for when your ASM have to jump deep into the enemy lines and it's upgrade can add overall dps and disruption to your entire army. So use it as often as possible.
Now that I use it more I have started noticing SO MANY SM players not dropping the banner in the engagements, the cooldown isn't too long and if you cripple your opponents forces it will delay their next attack long enough for you to get another one.
So in short, new players use banner as often as possible, just to get in the habit of knowing you have it. Eventually you will start recognizing proper situations where you can maximize the banners effect. Till then just drop it during every engagement if possible.