John Q. Ericson
During a group warm-up session I attended at the 2009 International Horn Symposium (led by Lydia Van Dreel) a participant asked a question about low range fingerings. This topic is one that is surprisingly hot, as it relates to the use of the F and Bb horn in the low range. Some teachers have strong opinions on the fingering choices.
I have observed that many amateurs and students play using exclusively F horn fingerings in the range from written F below the staff to C#. I would suggest that the default fingerings for these notes be on the Bb horn instead. If the musical situation is loud or articulated I would certainly use the Bb horn. If it is in a context where the music is soft and mostly sustained however I would consider the F horn.
Actually, in my studies I was taught the following system by David Wakefield that remains something of a personal default. He suggested when I studied with him at Aspen going down from the written low F to C# using these fingerings: 1, 3, 23, T3, T23. Intonation wise in a situation where these notes go by slowly these are great fingerings; the 13 and 123 combinations for low D and C# should always be avoided, they are very sharp and stuffy. However, if I have a more articulated situation where more volume is required I will certainly use the Bb horn, and note in my teaching that this in general works better for all my students as a default.
Above all you are not breaking some great rule of horn playing to use the Bb horn in this range. Any pro would do the same, professional low horn players use more Bb horn than many audience members would ever guess. In the end it is more about how it sounds in the hall rather than being true to some perceived tradition of fingerings for the low horn.