I think it's more important to keep consistency across the text. In HarryT's example, choose whether you want "today" or "to-day", and stick to that decision everywhere (I had to do exactly that recently with Galsworthy works).
Note, too, that by keeping the punctuation and spelling as in the "original", you are not always being true to the author, but to the original editor or publisher, as those decisions were typically not taken by the authors. But it can be different, Lewis Carroll wrote consistently "ca'n't", "wo'n't", etc., and it was
his choice, not the editor's.
So... it depends
