its usually whatever you feel most comfortable with.
i ride 15 wide, but whatever feels right for you.
its all preference, if you ride 18 now i would just stick with it.
I ride 18" atm, barspins are easy, 360 barspins too...
I'm going 20-22 next time I buy new bars...since I ride a BMX onece in a while (like...once-twice a month) and I miss the wide bars after I rode the BMX : )