6 wk old sleeps a 6 hour stretch at night and could go longer. Should I wake to feed or let her keep sleeping?

Wake. I don't like babies sleeping any longer than 6 hours at a stretch when they're that young. They just don't have the fat stores to keep their glucose levels up through long periods of time.
A Full Night. You have asked this question several times in different ways. The answers are all the same. Let her sleep!
Let her sleep. As long as she is eating when she is awake and wetting and filling diapers. Let her sleep. She will wake when she's hungry. Get some sleep yourself.
Let baby set schedul. Any 6 wk old that has been feeding & growing well can be left alone to sleep as long as 8 or more hours. Their intestines has food they are continuing to digest & use during this period & if baby can deal with it & thrive there is no reason to wake her. Babies don't sleep more than 4 hr without cycling to a lighter sleep state so the fact that yours can settle back & sleep longer is wonderful.
Let Sleep. I never wake a healthy baby for feedings. If you are fortunate enough for them to sleep an extended period of time and they are both healthy and gaining weight, enjoy. Let nature take its' course.