@stefan I don't think it's a big deal to mention your "via", and users who receive few interactions would actually be happy. I'd do it, and just refrain to do so if someone complains about it.
There's not going to be one right way to do it anyway. Some people love receiving a lot of likes, and some threaten to block those who "spam their notes". Not to mention those who think Liking without Reblogging is like a crime against humanity.
But still. I love Tumblr always an interesting place, and there's still some old-school-web vibe, in spite of being mostly inhabited by people who are probably too young to have experienced it.