Does using quippe in a relative clause require conjunctive?

Question

One can insert the particle quippe in a relative clause to give it a causal or otherwise explaining tone.¹ Does quippe require using conjunctive in the relative clause? If not, are there some rules for choosing between indicative and conjunctive that are special to quippe? Causal relative clauses usually have conjunctive, but I wonder if quippe is enough for expressing causality and the conjunctive can be omitted.

Other words can also be used to emphasize a causal nature of a relative clause: ut, utpote and praesertim. I suspect these behave like quippe as far as using conjunctive is concerned.

After reading my grammar once again while preparing this question, I am inclined to believe that conjunctive is obligatory with quippe in a relative clause, but I want to have my suspicions clearly confirmed or refuted.

This question originated from our chat room. When chatting about using three languages at the same time, I remarked: Sed crediderim id facilius esse Cerbero, quippe qui tria capita habet. Cerberus questioned my choice of indicative, and since I couldn't find a definitive answer elsewhere, I had to ask this question.

---

¹ I have only ever seen quippe in this use. If you know other uses, please answer this question. I have explained the causal use in my answer² to that question.

² I may have to update that answer depending on the answer I get here.

Answer 1

It seems that either mode was used, but each author seems to have chosen either one or the other, or both. Based on a cursory glance at the first couple of pages with quippe qui from the HP corpus, the following impressions present themselves:

Plautus uses the indicative almost exclusively.

Sallustius seems to use mainly the indicative, but also sometimes the subjunctive.

Cicero uses the subjunctive exclusively.

Livius uses the subjunctive in most cases, but sometimes the indicative.

Apuleius seems to be using both moods.

Etc.