Thanks for the thoughtful questions! Just a couple thoughts:
Answering the descriptive, not the normative: I use #didathing when
either (a) I’ve made a meaningful contribution that doesn’t fit into the
graph anywhere else, or (b) I want to signal-boost something that I’ve
done because I think that others may find it interesting or valuable.
An example of (a) is, “I spent some time trying out something that would have been great if it worked out, but wasn’t able to get it to work out (and it’s small enough that I don’t feel like writing something careful on Discourse)”.
An example of (b) is, “I documented some investigation in a comment in a
repository not under our organization (e.g., vsoch/sourcecred), so you
might not have been notified, but devs might like to read it”.
Again, this is just what I do; interested to hear from others, too.
My two cents: go for it. Sharing your work is itself helpful to the community. It gives people insight into what you’re working on and what you consider valuable or enjoyable. It gives people a chance to learn about a part of the project that they didn’t know existed. It helps us improve our bus factor.
As to perception of gaming, I wouldn’t worry too much, for three
reasons. First, our community operates at a high trust level, so a
priori I assume that people are acting in good faith. Second, this
particular form of gaming isn’t trivially effective. Posting in
#didathing just enables other people to use 
to mint and
flow cred to you. The extra step, involving human choice, acts as a
check, a form of multi-party approval. (I probably would advise against
-reacting to your own #didathing message, though. 
)
And third, I think it’s intrinsically important to the SourceCred
project that members of the SourceCred community follow what feels
natural to them. We’re dogfooding SourceCred on SourceCred so that we
can get a sense of how real people interact and behave when SourceCred
is live. Having a diverse set of practices from a diverse set of people
helps us underestand that better.
(We have a related thread about “When to 
something”, and I
wrote something similar there.)