Yeah, on page 122, his rules are
- Process the top item first
- Process one item at a time
- Never put anything back into "in"
but as I said, if you are getting the Inbox to zero on a regular schedule, and not spending a lot of time dawdling about trying to decide which item to process next, I'm not sure there is any great harm in doing it FIFO, LIFO, alphabetically, randomly, or by any other metric. I think his rules have more applicability to a paper-based Inbox than an electronic one where grabbing the 37th item is just as easy as the top one.
I usually start out by sorting the contents of the Inbox by Date Added (using the Edit->Sort feature) if I think there's a good chance I won't get through the whole thing. That way I'll usually do at least a few of the older items before breaking down and looking for the easy stuff!