Mandarin: tongue placement when pronouncing ü words like 學 and 去 that begin with q, x, j? Many Mandarin pronunciation guides suggest pronouncing the ü vowel with a tense tongue in mid-mouth as when pronouncing the "yi" sound.

However, for words starting with q, j, x, many sites suggest placing the tongue so it grazes the back of your bottom teeth.

For ü words that start with q, j, and x, how do you reconcile these tips?

Should the tongue graze the back of your bottom teeth, or should the tongue hold the position of the "yi" sound?
Use this guidance, it is entirely correct. This guy (John Pasden) has been the resident "foreign learner" on ChinesePod podcasts nearly from the beginning, for many years, before opening his own company.

Or on his other site, even with tongue placement pictures.