As far as I know, Washington is the only state that still centers the exit number legend, although as mentioned above, the tab is always full-width and installed as part of the guide sign. Every other state and agency aligns the exit number legend to the left or right, even if the tab is full-width and/or installed as part of the guide sign.
The exception to this rule are: Southwest Washington in general; APL signs; a few scattered signs elsewhere in state.