I have been reading on all the team builds etc. all over this site and I see where it looks like some people use different 300 point teams for each round. Also it looks like some people only use the new tank map. Whats the deal?

Well, you have to use the same team for each game... I assume people were referring to multiple games and/or what other players pulled and played in the game.

The "tank map" is the map that came in the Avengers set and is supposed to be used for the CoG events.

You can use only one team per tourney.
Only the tank is official, too.

...unless you run out of Avengers maps.... then you can use another map of the same size.

like i had to play in the prison map. grrrrrrrrrr.

... or if WK didn't send you any so the only Avengers map on hand is the one they Judge happened to have from his case for his own personal use

I don't think WK sent any with the prize support at all....
It happened to be at my venue that the store owner saved each of the maps from the cases he opened, so we had more than enough, but I know a good number of stores may not have had enough maps if the players that bought a case didn't want to bring theirs in.

The venue last week used the JSA museum because we didn't have any new maps.

You HAVE to use the same team round after round? Huh. Guess we didn't know that.

After opening your boosters and verifying the contents, each player has 15 minutes to build their team. That much is spelled out in the HeroClix Comprehensive Tournament Rules. The WizKids Approved Play Rules document is where it gets specific about your comment:

I have been at venues that have allowed changing teams between rounds in sealed events. My understanding is that the official ruling on this has changed back and forth.

Note as a separate point that there is no dictate from on high that CoG tourneys must be on the Avengers map. There have been very few unambiguous dictates from on hight about them at all, but lots of people saying that the tournaments must be run this or that way.

I've been playing since before Indy and it has never gone back and forth while I have been around. Maybe before that... though I doubt it. Once you construct a force, you construct a force. How would changing your teams from random boosters between rounds be any different than swapping out different figs from your collection during a normal, restricted event?

In a sealed event, you only get a few minutes to think about your force. Sometimes you put something together and only realize in the course of a game that the team just doesn't work. Being forced to play with an ill-conceived team for two more games after that can be a drag. Of course, you are still confined to your booster pulls-- so it's not like calling on the resources of your whole collection in the middle of a tourney.

At the venues that played that way, it worked fine. The only downside was that there were sometimes delays between rounds.

Regardless, I don't know whether the Approved Play rules really did ever allow for it or not. It doesn't seem like a big deal. Since everyone at a tournament has the same opportunities-- to change or not-- either is fair.

This reminded me that there was supposedly another map for CoG.

I guess it will be sent week 5 with Galactus.

The official ruling has never changed... you've never been allowed to 'sideboard' in any sealed events. Some venues might allow it, but it's never been part of the official rules.