すみません or 済みません, こんにちは or 今日は

こんにちは
I was wondering how "realistic" was the use of kanji for some basic expression like 済みません or 今日は. Is it good to study these forms or can it become more of an "intellectual" exercise with little or not impact on actual literature. I must say that I mostly read manga at the moment which usually simplify these forms but I hope to go up a notch soon and I have little knowledge of what to expect in these matters.
ありがとう.

kana is more appropriate for both. Every time I see 今日は I read it as きょうは, not こんにちは. In most cases, kanji is intended to increase the efficiency of reading. I think that it serves the opposite purposes in cases like this. It's similar to writing 有り難う御座います, when ありがとうございます is much more common and easy to read (at least to me)

Yes, most people write those words in kana nowadays, especially こんにちは. I remember reading a translation of this one 漫画家's freetalk column and in the original Japanese, she had said 「今日は、日渡早紀です。」 The translator translated this as "Today, I'm Hiwatari Saki", all because the translator misread 今日は as きょうは instead of こんにちは!