When I manually try to drag staves / parts apart to open up some space in my score, sometimes it works perfectly. However, at other times, I can only do this for so far and then the spacing goes super wide to fill the entire page.
1) Why does it sometimes 'jump' apart when other times it will allow incremental adjustments?
2) Is it best to concern myself with these changes after I have tried to hide empty staves?
You need first of all to experiment with the vertical justification settings under Layout > Staff Spacing > Staves > "Justify staves when page is at least xx% full". This setting is what results in the staves "jumping" when they are dragged more than a small amount.
Other settings to explore are Layout > Spaces between Staves/Systems.
As you change these, judge the overall effect on the score by highlighting everything and clicking on Layout > Optimize.
I suggest you do not drag staves around except as a final fine tweaking once everything else is sorted out.
As always, it's best to worry about the cosmetic appearance when everything else is done.
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Chris Crawley (composer and horn-player) using Sibelius 7.1.3, NotePerformer, Windows 10 64-bit, Intel Core i7 3.30GHz, 64GB RAM, DacMagic 100