Should I go to the ER for a mild uti?

No. Uti's are common. I would start with increased fluids and cranberry tablets, but if symptoms get worse or you develop fever then i would see your doctor in the office to get treated. I would try to avoid the emergency room.
No. You should almost never go to the er for something mild, since the er is for emergencies. It's better to see your doctor or go to an urgent care center. Unfortunately, uninsured people often have to rely on ers for routine health problems due to lack of other options.