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I wouldn't have thought you would need to treat I2C lines as
transmission lines, the data rate is quite slow (even at high speed
I2C you are still only at 4MHz). It is not as though you are laying
out a memory array where a heap of parallel address and data lines
need to be synchronised to a double data rate clock, and so edge
timings are highly critical. The I2C transfer clocking is designed to
deal with a small amount of ringing on the clock and data lines.
Considering it was originally designed to be run in a wire harness in
a TV set, it should be quite robust.

But as to track width on a PCB, as you are not constrained by
requiring impedance controlled tracks, make them wide enough that they
will be robust in manufacturing of the PCB (i.e. don't create
under-etching problems, corner peel problems, etc) so that the PCB
house minimises rejection rates, and then during assembly &
integration the tracks are robust enough to withstand knocks and rough
handling without causing micro-cracks in the tracks. Personally I
would go for something like 0.5mm, only thinning when needed to
squeeze through gaps between pins and pads.