All that's required is that the trap be above the floor. Keep in mind gravity...you don't want it too high. Nor, do you want it so low, you waste all space in the cabinet. The safest thing is to select the cabinet AND sink you want, then install the thing so you don't need to be a contortionist to finish up. Also, take into account whether you plan a disposer, as that requires the connection in the wall to be lower so things don't have to try to flow uphill (or keep standing water in the disposer!). There are 'normal' heights, but there are so many different sinks out there, it's often not safe to assume that will work in your circumstances.