[Tatsujin]

The difference between watashi and watakushi is more than just politeness. You have to concider a situational norm when using watakushi, since it can work both ways - being impolite and polite. Watashi is itself a very polite way of referring to yourself. Since in the spoken language you don't usually refer to "me" that much, watashi is a way to make yourself look "lesser" being in the eyes of the person you are talking to, or to make the listener feel "above" you. Basically watashi is used when ever talking to a new aqcuintance or a person of elder stature to you.

Watakushi is normally not used in speach what so ever. Watakushi is concidered sometimes to be too polite in the modern japanese. However when presenting, for example a thesis, to a group of listeners significantly more important in the taken situation than you, watakushi can be used. It is however strikingly "too polite" in most cases.

Watakushi can also sound too polite in a way that the speaker, intentionally or unintentionally, makes him/herself look too "low" in the eyes of the listener. For example if you use watakushi when speaking to a person of same level to you, like a fellow student, you can sound insulting. You raise the politness level too much for the listener at hand. To be frank, in this case the speaker would sound like the biggest smart-ass.

Boku and ore are so called slang words. Boku is a very neutral way to address to yourself, but in some regions it is concidered to be somewhat femine. Ore is the most impolite way to say "me". It should be used only when talking to a person of same level as you, who you know well. A basic rule is that you should never use ore if there is even a slightest doubt that the listener doesn't feel like he/she is at the same level as you. Situations to use ore could be when talking to a family member or a long time friend that is of same age and position (i.e. student) and when there are no people of higher "rank" listening.

The rule of thumb: If you are not 100% sure that you should use watakushi, don't. If you are not 100% sure that you could use ore, don't.

Hope this clears it.