The careful writer studiously avoids any situation that would violate the standard rules of English orthographic convention in this regard. That entails resorting at need to reärrangement or even the circumlocutionary introduction of a tractable word to begin the sentence.
This regularly arises in technical writing when discussing case-sensitive literal variable and function names. So instead of writing:
This is the previous sentence. fork makes a copy of the current process.
And violating the rule of capitalization, one writes:
This is the previous sentence. The fork function makes a copy of the current process.
This not only looks a lot better, it will never be received with confusion or scorn by anyone — as in fact the first version often enough is.
Similarly, do not write this:
This is the previous sentence. iTunes costs too much.
When this works better:
This is the previous sentence. The iTunes program costs too much.
Yes, it’s fussy, but no one will look down at you for doing it that way. The other way, some will.