IMO unless you brake hard enough (emergency stop) your not going to know if your rears lock up before your fronts, and as a rule most of us dont brake that hard day to day, but if they do it's like pulling your handbrake on at whatever speed you happen to be doing!
My car came with the rear disc conversion already done but if it was my choice i'd of stayed with drums and just fitted bigger front brakes.
As it is now i'm fitting 300mm Focus ST front brakes and going to see if that balances it out enough for the fronts to lock before the back (and i wanted better front brakes anyway), if it doesnt then a bias valve will be next on my shopping list.