Further to my snide remark over on the
Brendan McCarthy's Spider-Man: Fever thread, I thought it might be as well to post something useful on the subject.
First of all ...crossbar 'I's, a reminder:

There is a school of thought that says it's also appropriate to use crossbar 'I's on the first letter of proper nouns (like Ian or Ingrid), which I have no problem with. Many lettering fonts also have a crossbar and a plain J, the crossbar J technically only being used on proper nouns as well:

In most lettering fonts, this the crossbar version is the uppercase character, and the plain version is the lower, and this is where the problem usually arises.
Some writers, since they are used to seeing their stories lettered in all caps, type their scripts accordingly:
DREDD: DROKK IT! THIS CREEP'S GOING
DOWN!(Note: if you're reading this, and you're a writer, and you do this, stop.
Please.)
Back in the old days of hand lettering, this made the script easier to read. Nowadays, the script is usually e-mailed to the letterer, pretty much always as a Word document, so that the letterer can copy and paste the text onto the Illustrator document.
Even if the writer has used standard sentence case, there will still be uppercase 'I's in the script where you -- as letterer -- don't want to use crossbars.
For example, in the
Spider-Man: Fever preview pages there's a crossbar I wherever it's been typed as a capital in the script, so if a sentence begins "In another ..." as it does on the first story page on that link, they've put a crossbar I when it should be a plain one. There's one in "New York CITY" and another in "SPIDER-MAN" on the second story page, as well.
There are several things a letterer needs to do to a script in Word before starting to transfer the text over to the actual lettering document.
If the script is ALL CAPS then the first thing is to go Edit -> Select All, then Format -> Change Case -> Sentence Case.
Next, you need to get rid of all sorts of typing quirks that may have crept in. Use Find/Replace under the edit menu.
Find: __Double Space (literally type two spaces where I've used underscores)
Replace: _ Single Space (literally one space instead of the underscore)
Find: ... (three full stops)
Replace: … (ellipsis)
Now, for those 'I's. You need to have "Match Case" selected to make this work.
Find: I (capital I, no spaces before or after)
Replace: i (lower case i, no spaces before or after)
Find: i_ (lower case i with space after)
Replace: I_ (uppercase with space after)
Find: i' (lower case i with apostrophe after, no spaces)
Replace" I' (uppercase I with apostrophe and no space)
Your script should now be broadly lettering friendly (although keep an eye open for words that END in 'i', because you will have stuck a capital I at the end of 'taxi', for example).
EDIT TO ADD: You could probably stick all that on a macro to save time, thinking about it.
Also be aware that Word will substitute a double-dash for an em-hyphen. In British comics, the em-hyphen is preferred, but in the US, the double-dash is used exclusively. You can turn off the substitution in Word's Autocorrect options.
Finally, don't think that you can't EVER use a crossbar 'I' anywhere else -- it's fine as long as you're using it deliberately, and are trying to achieve a specific effect ... if you're going for an old-school Marvel Artie Simek/Joe Simon look, for example:

Cheers!
Jim