On a more serious note, there are quite reasonable properties that are satisfied by keeping the sign of the first argument as the chosen representative. In particular, for any two nonzero integers a and b, we have that:

a = (a / b) * b + (a mod b)

Where division should be interpreted as integer division that rounds towards zero. If a=-1 and b=3, then (a/b) would be zero which would require (a mod b) to be -1 if we want the above to hold. Also note that other rounding choices (e.g., rounding towards negative infinity) could impose (a mod b) = 2.

So choosing a particular representative comes down to choosing what properties you want your function to have in relation to other arithmetic.