When I was trained in Orton Gillingham, I was trained that "you use an <-es> after <s, ss, z, zz, sh, ch, tch> and <x>."
That's a lot of memorization for kids with crappy phonological memories, isn't it? It's also inelegant, and incomplete. It is inelegant because it multiplies entities unnecessarily, and incomplete because it fails to account for other spellings of sibilants that would also require an <-es> instead of an <-s>, which I'll detail below.
The question about <-s> or <-es> was posed in a Facebook group, and the bad advice commenced, very focused on how to teach this to others and far less on how to understand this for oneself. The bad advice includes the folllowing:
1. Go read about a 5-day investigation I did with my kids.
2. Buy the Tool Box for $300+ and you'll get an explanation.
3. The additional syllable is added whenever the final phoneme in the noun or verb ends in an alveolar or palatal fricative.
#1 and #2 are pretty useless. This doesn't require a long investigation. #3 is useless for different reasons: (a) the terminology is superfluous, and (b) English doesn't have any palatal fricative phonemes. Womp-womp.
The actual understanding of <-s> and <-es> is clear and concise, and if studied accurately, reveals something deeper about the way English writes its words.