You know, I've seen you write this before here, but I still don't know why. Why would this be a bad practice?
Sure, there will be some minor variations between diodes even of the same type. But in a parallel string, each LED can draw whatever current it needs without affecting its neighbors, and the overall current-limiting resistor should still limit overall current for the string to a safe level, right? So far as voltage goes, there may again be some minor differences between forward voltage drops among LED brothers/sisters, but again, I don't see why this would be a show stopper.
In fact, by putting them in series, aren't you in more danger of overdriving the "weaker" diodes with excessive current from the "stronger" ones?
Or am I missing something really obvious here?