I haven't found it to be an issue as long as the sights will zero. So the rail needs to be mounted up square with the receiver. I suppose there could be issues with the rail flexing under outside pressure in relation to barrel position, but it takes a fair bit of pressure to accomplish. I have seen video of the Geissele rails being very resistant to flexion issues.

The most durable set up is to have the front sight mounted to the barrel via pins. Makes rail installation tricky tough, and not nearly as many options that don't stop at the front sight. The RIS II and Centurion are the only ones that come to mind.