Signed to unsigned for registers
When you store a value in a hardware register you are storing the bit representation of that value in the register. What effect that will have is entirely defined by the particular register.

In a lot of cases hardware registers contain either an unsigned value (counters, (pre-)scalers, A/D-result) or a set of bit fields. It is generally easier to work with such values as unsigned.

Occasionally you will find a register that has its bits in a 2s-complement form (fi. in an A/D converter with a differential input), in that case it will be more appropriate to load it from or to a signed variable.