interesting stuff... I use technorati and stat check; and it helps me see WHO is linking to me, even in a post... I like to know; and I have in one case asked someone to remove my link; as their blog contained content I was uncomfortable with and didn't want to appear to endorse.
But because I am constantly checking I have rarely asked someone if I could link to them; And I link out to a TON of people! I figured others were also checking their stats and link backs too... I always just thought someone will let me know if they didn't want to be marketed via link from my blog and if they wanted me to remove the link I would; without being offended.
I would however ALWAYS ask someone before I used a blog link in a post... like a featured stamper or something. This has happened to be a couple times; and while I was flattered; and would have likely given my consent, I was disappointed I was not asked.