Linking old posts helps all of the new readers who are following the conversation; this is probably more important than any effects on the person you're directly responding to.

Always err on the side of littering your comment with extra links. IME, that's more practical and helpful, and I've never personally felt irked when reading posts or comments with lots of links to basic Sequence material.

In most cases, I've found that it actually helps remember the key points by seeing the page again, and helps most arguments flow more smoothly.

There is no balance. It's always better to provide the links.

If you got a reference in mind, linking it will always be more helpful than not.

The only failure mode to avoid is implicitly or explicitly stating "Because you haven't read X, your input is not worth considering."

There was a time when that was a common failure mode on LW ("Go read the Sequences, then we'll talk"). Less so now.

I generally take a moment to think about how relevant the Sequence post is. Most of the time, I conclude that <10% of the post is actually relevant to my point, so I don't bother linking, as it seems like it enormously diffuses what I'm trying to express. (I don't link nominally relevant wikipedia articles for similar reasons.)