Good vs. Evil, Law vs. Chaos
Dungeons and Dragons has always had these two eternal battles raging. The deities are aligned. They are good or evil, lawful or chaotic, or some point in between. They live in domains permeated with these ideals. The deities and other residents of these domains could be considered the embodiment of these ideals, formed themselves out of the primordial good and evil, law and chaos.
Creatures of these outer planes have significantly more power when in their own domain and significantly less in an opposing domain.
There is no denying the alignment of the outer planes or of the denizens thereof.
But what of characters? Do (should) characters have a stated alignment? Certainly all characters have a set of morals, a code of ethics that they live by. They are different though for everyone and even the definition will vary from person to person.
This is a question that has plagued D&D since its inception. Should we "bother" with alignment?
What's The Point
If someone calls themselves lawful or chaotic that doesn't mean that they are. If they call themselves lawful but act chaotically, are they punished? Most characters call themselves "good" yet they wander the countryside looking for things to kill for profit and gain.
What about Clerics and Paladins?
Deities bestow powers onto their followers. If a follower doesn't adhere to the ideals of the deity, it makes sense that the deity would stop bestowing powers. I would expect that the deities would have some flexibility though. For instance, a Lawful Good deity may accept followers who are Lawful Neutral or Neutral Good. Paladins may be more restrictive but that would be up to each deity (or DM) to decide.
But is it necessary for a character to declare an alignment?
The way in which a character is played shows the character's alignment. In my opinion, that is where it should be left. For most characters, it does not matter what alignment they are. If a character is being evaluated by its alignment, they will be evaluated by their previous actions not by what they profess.
If a character is about to knowingly do something that would endanger her standing with her deity, the DM should tell her so and give her the chance to rethink her actions. If she goes ahead anyway, then the DM needs to determine how the deity will respond.
It wouldn't hurt for the DM, from time to time, to let players know what their perceived alignment is based on their actions. Each person has a different definition of the various alignments and since the DM's definition is all that really counts here, players should know where they stand based on that definition.