Starting this up since it seems like some folks may get some value from it (and it will help them memorize). It is a work-in-progress at the moment and I'll be updating as I get more screenshots of each treasure room for each map. If you'd like to contribute any shots I'm missing so far, throw them my way and I'll put them up! I am looking for two shots of each door: the location on the map, and the general interior of the room itself.
Treasure rooms must be opened by specific classes: stone doors can only be opened by warriors, golden locked doors can only be opened by rogues, and purple glowy wards can only be opened by mages. You do not have to open these doors to complete the match, but they can award money or an item card. There are six treasure rooms in a match.
In these treasure rooms is a chest and some money pots. Some rooms will NEVER have a guardian to fight after you open the chest, so we tend to call those "freebies". They are 100% safe to open. Other rooms have a chance to spawn a guardian when you attempt to open the chest. This comes in three forms: a round of corpses (relatively easy and usually confined to Zone 1 treasure rooms only), an Arcane Horror (not too bad, but can damage you pretty quickly if you don't avoid his spells), or a Revenant (the toughest guardian who can grab you, pull you in, and basically kill anyone who isn't a tank).
The general rules of thumb for quickly identifying a safe room from a potential guardian room are as follows:
1. Any treasure rooms that already had enemies inside when you opened the door will not contain a chest guardian.
2. Smallish rooms that are also sort of awkward-shaped most of the time will not contain a chest guardian.
3. Any chest that's in a corridor-area where you pull a lever to open a door and proceed further in the level via an alternate route instead of the "main" route will not contain a chest guardian.
4. If a treasure room contains money pots, it is NOT safe and the chest could spawn a guardian. Treasure rooms with no pots inside of them are totally safe. - Thanks to Shiratori for that tip!
I will now attempt to document all treasure rooms in the game for your convenience. Rooms that have a chance to spawn chest guardians are hidden under spoilers to help conserve space.
Elven Ruins (Destruction DLC Alternate Path Version):
For these two, the one on the left always has a chance to contain a chest guardian. The one on the right leads down a rubble pathway and is always safe. The chest for that one is on the left and an exit gate back to the main level is to the right.
The "room" on the right:
This one is reached by climbing up a small hill. The level continues to the left and to the exit door of the zone. To the right of the hilltop is a treasure door that is always safe to open.
Elven Ruins (Normal Routes):
Added as of 4/20/2015:
Near the start of this zone, you'll run ahead and see this door in front of you (and another way off to the left down a pathway). This one is always safe and leads to doors (either left or right) that go downstairs to a mob.