No. You don't need to buy a "US version" of the game if yo have a US account. Just download the client from
www.guildwars.com and log in.
If you buy a Chinese, Korean, or Japanese CD Key and want to do fun things such as make cute guild tags or character names with double-byte characters, you need to register an account on the NC site corresponding to whichever key you got. In your case, you would register a new NC Taiwan account (as opposed to adding the key to your North American NCSoft account which you can access and modify at
www.playNC.com) because keys from other regions will only work with NC accounts of the same region.
Hopefully this gives you what you need. If you have questions about
my personal experience using foreign CD Keys for Guild Wars, please message me. If you want further information about using foreign keys in general please use the search feature because I know I've seen this question here before.