The archer's guild teleporter is near the archer's guild, in the south west of Rivertown, near the entrance to the Dark Forest. The council hall and the teleporter there can be reached after you have been to the castle, get out and do another couple main plot quests. The rest of the teleporters you listed are in the Dark Forest.
Hearing enemies from a little further away can give you an idea of what is ahead sooner. It certainly wasn't my most sought after bonus, but it can be occasionally helpful.
If you have equipment that is good enough you think you will be using it for awhile (dragon armour, etc), I would only use silver or gold charms. I saved most charms until I had better equipment, but by the time that happened I didn't really need the extra bonus. I used 1 gold strength charm (a lucky drop by the dragon rider that Zandalor kills south of Aleroth) in my Amulet of Glory (from the catacombs) when I explored past the castle and fought the knight that drops a two handed bastard sword (I needed to go up a level and use the charm to meet the requirements).
At the end of the game I had 66 gold / very large charms (6 from before entering the final area; the rest were about half loot and half purchased), 59 silver / large charms, 24 red / medium charms, 19 blue / minor charms and 1 green / weak charm.
