To put it simple, current state is:
Toolbar operator butons:
- align left, if they have no icon -align to center, if they have icon
All other areas:
- align to center, no metter if they have or haven't icon.
- if icon is present, the alignment is offset to right from the buttons that don't have icons. If these are above/below, things get out of alignment
rest of the UI aligns everything to left in most cases.
possible solution 1 :
Align to left
if an icon is present in a column of buttons under each other, offset by icon width, also for buttons which don't have an icon.
this is consistent with menu drawing in 2.78, where icons aren't everywhere too.
possible solution 2
drop icons for operator buttons completely. It is a question if these are actually of any use except for add primitive panel, where they are used a lot. Icons in operator buttons are actually so rare they often look like a mistake, or taking an operator out of context.