If you're putting larger brakes on the front, you should be manually adjusting your bias anyways.
If you did them at the same time the stock bias(er) I assume would work fine if (and this is a big if)... if the surface area of the hydraulic contact to the piston was the same for both brakes. This is true for the stocks I believe (again I assume).
Let me explain... Assume a neutral bias, and equal length lines. All 4 brakes get the same pressure and flow when the pedal is depressed. Now throw brakes on the front with a larger surface area piston (lets assume that it takes a larger volume of fluid to displace the piston the same distance as the stock brake). When you press the pedal, the pressure is less, so the braking force is now more biased to the rear.
When you consider that the brake bias is somewhere around 65/35 (front/rear) and you add a larger brake in the front, the bias becomes more neutral because of what I explained above. This is bad, because say for example the bias is now 60/40. The rear brake is working more and the front brake less than stock. Less than stock?!?!, but you put bigger brakes on there for MORE stopping power. This is why you should replace the stock bias(er thing) with something adjustable.
I wrote this pretty quickly so let me know if i screwed something up.
