When practicing 3-note major 251’s is there a reason why in the left hand sometimes you go up from the 2 to the 5 and sometimes down?
Any problem with just playing the 2 with 1-7 shell voicing, the 1 and 3 of the 5 and then the major 7 shell of the one which is how I learned this before?
Hi Wendy 
Good questions!
No there is no reason behind this, other than that we have the freedom to voice our chords in different registers of the piano.
When playing a jazz standard, choosing which bass movement to use could depend on the melody note. For example, if the melody is around or below middle C, then perhaps it would be more appropriate to move our bass note down. It depends on the context.
The key takeaways from the major 251 lesson should be:
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understanding that the 251 is the most common progression in jazz music
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understanding the quality of the 3 chords in a 251 which is ii-7 to V7 to Imaj7 … Being able to spot this common sequence will help to read and interpret lead sheets.
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being able to visualise the b7 falling to the 3rd of the next chord. This is the most important point. This ‘half step voice leading’ is what creates smooth harmonies and melodies and so being able to visualise that in all 12 keys is invaluable.
That’s a great exercise too. There’s certainly no problem with that.
As we cover in the upcoming lessons and courses, there are many ways to voice 251 progressions.
The nice thing about having the bass in the left hand, and the 3rd and 7th in the right hand is that it isolates those 2 ‘guide tones’ and the voice leading that occurs between them.
I’d recommend doing both, your left hand shells, and also the method in the lesson.
The major 251s are one of the foundational building blocks of jazz harmony. Spending the time to truly absorb the 3-note voicings - in any variation - is a very worthwhile and productive exercise.
Hope this helps and any other questions just let us know.