If you just burn an FLV file (HD 1080p or SD) to DVD or Blu-Ray disc, nothing happens with its quality.

If you need to burn MPEG-2 videos to a DVD-Video disc you can choose between software that converts the videos (thus losing some quality) and software that can do it losslessly (we suppose that we don't need to change the video resolution when burning).

A similar situation is with Blu-Ray discs. If your FLV files use MPEG-4 AVC video codec, you can create a Blu-Ray disc with MPEG-4 AVC video without any conversion. However some programs will lose some quality during the conversion (even if they don't create a Blu-Ray disc using MPEG-2 or VC1 video codec).