Actually if you have some solid healers (which my teams have, most of the time *cough*)
- it doesn't matter much where you fight, as long as you're sure you're keeping at least as many destros occupied as you committed yourself. The trick is to make sure your runner(s) have enough space to grab that banner and leg it.
So far I've encountered 3 possible scenarios:
1: Defense at your own flag.
The surest option, but funnily enough I don't like it much myself. Reason for this is that you don't control any of the map, making it a long walk for your own runners
2. A slagfest in midfield.
Happens most of the time. When you're healing-light make sure you're on your side of the bridge, so that you're out of range of the bolt throwers.
3. (When you're stronger than the enemy:) pin them on their own base! Drawback here is that actually grabbing the flag will be more difficult due to AOE.
Also, their runners may grab your flag and try to run it. However, if your team really is stronger, you'll be able to keep 90% of their team on of near their respawn.
One or two dedicated defenders on your own flag will dissue any enemy from trying to get it, and with so many of your own team grabbing at the destro flag it will eventually be picked up - and they should have a free run back to home base after that!
Needless to say, I prefer 3 myself. However, as healer, I liked 2 better than 1 - imho 2. is only the "stupid" option for the team committing most resources to the melee!