Believe it or not, under most cases it's more of a philosophical point than an engineering one.
With the exception of "X" braced frames, they'll have the usual zig zag bracing. You can have the bottom diagonal brace go from bottom-to-top or top-to-bottom facing the aisle.
From DAK Equipment:
"With a bottom-to-top orientation, the diagonal brace absorbs tension and adds support if the column is damaged. Depending on the point of impact, the diagonal and horizontal braces and welds add stiffness to the column and can reduce the damage from an impact.
A top-to-bottom set-up facing the aisle has the advantage of avoiding breaking the welds of the diagonal brace upon impact and further reducing the capacity and reliability of the frame.
According to the Rack Manufacturers Institute, unless the orientation is design-critical, either option works well and is simply a matter of personal preference."
Why not add rack column protectors and get more sleep at night?