Yeah, I spent about 20 seconds thinking "is there a database of #5 starters? No, because I don't even know if half the teams had a consistent one." Detroit's a great example of the "#5 starter" being a myth - they bullpenned a prototypical #5 (Coke) for a trade acquisition, so there's nobody who had the spot with regularity.
And I think you'll find that's the case with most teams, though, right? The #5 starter is the least consistent starter on the staff, with the spot occupied by the person who is shuttled between AAA/MLB, injured, etc.
The mythical "#5 starter" for most teams, I'd bet, is a combination of 3-4 pitchers who average around a 4.50-5.00 ERA.