What should the head speed set at on a MX450 XS for stock and carbon fiber blades?

Mine's turning 2800 and flying great with MAH carbons. I've never been able to get the stock blades to track well (glass or carbon, either), so no idea about them.

Can you sand carbon fiber blades to help with balancing?

Probably, but I've never heard of anyone trying. The usual method is to add weight, usually using clear packing tape, rather than subtracting. Subtracting obviously has its appeal, but since the blades are a layered composite construct I don't know if it would work as it does for fixed-wing propellers. Maybe for small adjustments?

Do you put the packing tape around the blade or just down one side?

The main thing I do when I put tape on is put it in the place that is least likely to mess up the airflow which would change the lift. You could try sanding the blades also, but it might be hard to get back to that nice polished look that CF blades have, and you would probably need to seal where you sanded with some sort of something, like what holds the CF together. CA or some sort of epoxy would probably work (maybe someone else has more info on that)

Where placement is concerned, the two things two consider are if it comes loose and the balance of the blades as a whole. For the first, just make sure that the edge of the tape faces to the rear of the blade-- usually means wrapping it over the leading edge, but if (heaven forbid!) you have to put on a piece that goes all the way around, just make sure the overlapping edge goes to the rear. This keeps the airflow from lifting the tape.

For balance, the general idea is to find the center of gravity of each blade spanwise, then add weight to the lighter blade in the position that will move its center of gravity to the same position as the other at the same time as it equalizes the weight of the two blades. This is often easy in my experience-- the spanwise CGs of decent blades is usually pretty close to the same spot on blades in the same pack, so you're adding weight centered on the spanwise CG of the blade. There are a lot of threads on balancing that go into this in great detail -- try looking at SSG Scott's posts; he has a good PDF on balancing with scales, and other info on balancing the other parts of the drivetrain.

On larger blades, you also need to balance chordwise to match the chordwise CG to the center of lift (ideally) or at least to consistency between the two blades, but this usually isn't a problem with 450-sized helis for general flying. For spanwise balance you can use a variety of tools; for chord- and spanwise, you use something like a Koll (I don't have one, just know of it).