Bb or F side? What should you practice?

In my opinion students should practice scales & most etudes in three ways.

1. Everything on the F side
2. Everything on the Bb side except for notes that can't be reached on the Bb side
3. Whatever sensible mixed fingering is convenient and produces a good tone.

For the mixed fingering, G and F# should of course more or less always be on the F side unless it result in awkward fingerings. As for F downwards, it depends very much on the player and the instrument. Personally I tend to use the Bb side in the whole F to C# range, as I find the notes blow more freely on the Bb side on my horn. But if I want a particular velvety tone for a soft
entry, then I may switch to the F side for the purpose.

But it is necessary to know your scales on both sides and be thoroughly familiar with both sets of fingerings in order to be able to switch easily for whatever purpose you might need.

For instance if you work on the principle that everything from 2nd-line G downwards should be played on the F side, and everything above on the Bb side (a fairly common recommendation, but which I do not endorse), then you may have some difficulty achieving a clean slur up a 4th from G to C. But that slur is fairly easy if it is done with both notes played open on the F side. Once you have learned not to hit the intermediate Bb harmonic on the way up, you can get a beautifully clean slur, much cleaner than you can get when moving the thumb valve, which inevitably results in the air having to start resonating in a long new length of tubing. Even more difficult would be a slur from F to Bb, which would be cleaner either as a lip slur on the F side, or a 0-1 slur on the Bb side.

But these alternatives aren't available unless you have sufficient facility on both sides of the instrument to take advantage of them. Therefore, you have to achieve more or less equal familiarity with the fingerings of both sides of the horn.