Is it alright to use lowercase "i" or should you always use "I" (uppercase)?
Solution 1:
It is the standard orthography of English to capitalize the first person singular pronoun, as well as in contractions like I'm or I'll. This is not a universal property of written language, though—far from it.
Apparently the capitalization of I comes from England sometime before the time of Chaucer. The typographists of the day dictated this change; they thought that i (after being truncated from something more German-like "ich") was simply too small to stand on its own and bear so much meaning. Just goes to show how much of a technology writing really is.
Solution 2:
The orthography is what the orthography is, and while there are many variations allowed in certain aspects, no serious authority supports abandoning the distinction between upper and lower case. So unless you are sticking with a rebellious all-lower case spelling, a lower case "i" is always wrong.
Solution 3:
If you're looking for justification outside of a dictionary, there are countless style guidelines that will address the issue -- you'd think. I can't find much, at least not online.
"Me, Myself and I" -- New York Times article on this very question. (It comes to the same conclusion -- there's no real grammatical reason for capitalizing "I" aside from typography.)
Solution 4:
Well, it really depends on what you are writing. If you are writing something formal, or if you think your audience will care, then sure. If you are texting or IMing, or being much more casual, then it's really up to you whether you want to bother or not. Capitalization, like most orthography, is just a matter of convention.