Back in the old days, the fear was that the glass would sag and some analysis was done and the 1:6 ratio was deemed to be sufficient for mirrors. Now that we have better tools in the hands of the less math intelligent, it's possible to do better studies of deformations in glass and it's been found that the old timers were fairly conservative in their calculations. It's now typical to see mirrors in the 1:10 as more normal and even higher ratios for use with higher support point mounts. The professionals are doing mirrors that are so thin that you might as well throw out the ratio measurement system.
All it takes is a bit more care not to bend things as you make them and to not allow the mirror to take on shapes that are not desired.