I've found that right combination of display: inline-block, position: relative, positioning and margins will get it where you need every time.

What happens when a user agent overrides your preferred font size?

Serious problem. I usually have my page magnification jacked up to reduce eye strain, and it's amazing how many things go wrong on the pages I browse. Don't use a clever hack without testing how it looks after a few cmd-+ (ctrl-+) resizings.

Just don't use font-size and line-heights that references pixels for the <span> containing your arrow and you should be fine. It might not be perfectly aligned, but it won't break.