Great question. The reason I left align it is to maintain the left padding AND the right padding. My text symbol in my button is set to Resize, which means it will maintain the right margins/padding on all edges. In my base symbol, the text is 8px on the left/right, and 6px on the top/bottom.
When I change the text of my button, I just have to resize it bigger or smaller, just up until the point that the text no longer wraps. At this point, the button will have perfect amount of margin/padding on the left and the right side.
Here’s a quick GIF. At the end of the GIF, I’m detaching the symbol, just to show you how the padding is equal on both sides.