I think missing or randomized timestamps would be fine. I imagine in many such cases, backlinks (replies, digs) would offer some meaningful idea of the time a message was published/replicated - as would, in some cases, looking at previous or subsequent messages in the author's feed.

patchfoo sorts posts by received time on /public, /private, /mentions, /live, and channels. (On all except /live, this can be changed to sort by claimed timestamp using ?sort=claimed). In a message thread page, ssb-sort sorts messages by causal order + timestamps.