If you're starting with a bare 60' x 60' concrete pad, your roof might be stronger with the posts set on 7.5' centers and one truss per post/column.  This is the norm around 'here' by several builders.  We have 2 built this way, one - 60'w x 120'L x 18'h, and one -54'w x 75'L x 14'h.    Both with 4-12 pitch (works good 'here').

The purlins will sure have less stress on them from the shorter span between trusses.  Both of the above sheds have 2"x4" purlins 2' on center, on edge, nailed to the top of the truss.  16' - 2"x4"s and overlap the ends.

Sidewalls with posts on 7.5' centers will put more strength in your walls as well.