When I make only small modifications, I try to adapt to the current coding style with respect to indentation, naming of variables (CamelCase or no_camel_case or whatever), placement of else (on a new line or not?) etc.

When the code is horrible, for example no indentation at all (or only on two levels :/), one-lettered variable names etc., then I make my code look ok, and refactor the rest according to my time constraints.

Of course that depends on whether I'll end up maintaining the code - the more I have to deal with it, the more it will converge to my notion of programming style.