Next Farkas in The Art of French Horn Playing briefly presents notes on puffing out the cheeks.
Conventional wisdom
Farkas felt it was to be avoided because it is “an indication that something is seriously wrong with the way the embouchure is being formed. …the student can be sure that he is not bringing the proper muscles into play.”
Gunther Schuller in Horn Technique also recommends that hornists “should also guard against letting his cheeks puff or allowing air pockets to form behind either the upper or lower lips.”
Reality is, it can be OK
However, after years of teaching I am really unconvinced that slight puffing out the cheeks is a problem. It may in fact help get your embouchure in the right position to play a given note in a given range at a given dynamic, thinking especially of the low range. If it sounds good, the approach is correct for you. If it does not, then you have to explore options. Keep an open mind.
Put another way, can you puff out your cheeks at all? If you can’t your face may just be too tight.
Every rule has an exception
And I am not the only one to note this. Frøydis Ree Wekre in her great book Thoughts on Playing the Horn Well correctly explains that
…every rule has an exception. Some have discovered that a little (controlled) air in the cheeks can be useful in special situations.