Actually I was more wondering if it's normal to put it into kanji. For example, you never see ありがとうございます as 有難う御座います。If you were writing a letter, would you yourself put ぼく as 僕？ That's what I'm wondering.
I actually came across some statistics recently on the percentage of print advertising using kanji, hiragana and katakana for various name references, Sacred Blue. It was quite interesting so will try to find that a link to that study again and post it this time.
If you want to write Japanese sentence politely, I suggest that you should use a lot of Kanji. Because the sentence that is written a lot of Kanji looks politely. But it's a stiff style of writing. If you want to write friendly, I suggest that you should not use a lot of Kanji. I think you don't write a love letter in stiff style of writing.
But the most important is the text of a letter but not Hiragana or Kanji.
If you have trouble displaying Japanese characters on web pages, try right-clicking the screen, choose encoding, and select Japanese Auto JIS. That may solve the problem of getting gibberish on web pages instead of kanji.
Speaking of which, I remember reading about common personal pronoun co-occurrences. They were as follows: watashi -> anata
boku -> kimi
ore -> omae
Of course, the actual usage depends on the situation, but I believe that these were paired as politeness equivalents (or at least approximations thereof).