One thing that you don't want is a guide flag that is too loose in the holder. A loose guide flag will not stay perpendicular in the corners making it more likely that the car will de-slot. It would be nice if all guide flags were a perfect fit in their holders, but that is seldom the case. I like to use guide flags that have a screw and washer as a retainer. I can adjust the screw so that the guide will stay perpendicular, but still turn smoothly, but that rules out having the guide spring loaded. Some cars use springs to keep the guide centered, I have never been able to do any testing to see if there is any advantage with respect to handling to that arrangement. One advantage might be that in a live marshalled race it would be easier to get the car back in the slot. My cars almost always have aftermarket wires and I arrange those so that the guide will be centered when it is out of the slot.