When the first and last notes of a beamed group are on the same staff degree, the beam is always horizontal, as follows:
However, under the following circumstances note groups beginning and ending on different staff degrees customarily bear horizontal beams as well.
a group of three or more notes in which all of the notes are
on the same staff degree except the first or last one,
provided the different note is furthest from the beam
Note that if the different note is closest to the beam, the beam should slant.
the note group is composed of a repeated pattern
the inner notes are below the imaginary line above the first and last notes
This can generally be achieved in SIBELIUS by going to Engraving Rules > Beams and Stems > Beam Positions, selecting “Optical beam positions” and changing the setting for “Horizontal if middle notes intrude by” to 0.5 space.
In FINALE, go to Document Options > Beams > Beaming Style and select "Flatten Beams based on Standard Note”.
This having been said, in very repetitive music you might consider making all beams horizontal. In other words to do might want to do this:
instead of this:
To obtain this result in SIBELIUS, go to Engraving Rules > Beams and Stems > Beam Positions and set all of the “Default slants per interval” to 0.
In FINALE, go to Document Options > Beams > Beaming Style and select "Flatten All Beams”.
For further information on this issue see:
Gould, Elaine. Behind Bars. London: Faber Music Ltd, 2011 — pages 22 & 23
Ross, Ted. Teach Yourself the Art of Music Engraving & Processing, 3rd edition. Miami Beach, FL: Hansen Books, 1987 — Chapter III, section 6