You should ideally channel all character/encoding handling through one channel which can be tested and validated once. If there's a chance that every single page and widget may behave differently with regards to encodings, you have a bigger problem. You want to nail encodings once, then concentrate on other problems.

Having said that, more tests are hardly ever bad. Only if you start obsessing about and testing the same thing over and over I'd start to worry about some root causes.