There's no real rule on that order. SD footage is going to have a lot of noise in it, that's just the nature of what it is and depending on the format of the SD, you'll have more or less noise.
My suggestion would be to grade it first. If you start crushing the blacks down a bit, you'll probably knock out some noise naturally. Consequently, if you raise the whites and mids, you'll add more noise. So if it were me, I'd grade everything first and then probably throw a single de-noise over the entire timeline.
Walter Biscardi, Jr.
Editor, Colorist, Director, Writer, Consultant, Author, Chef. HD Post and Production Biscardi Creative Media