Sometimes websites you want to include will have information about linking to them on their site. SinC wants you to ask permission to use their logo, but I didn't see anything about a plain link. I didn't look at the others you mentioned.
If I can find an email for the people I want to add, I write and ask if they would mind my putting a link to them on my site. To me, it just seems more courteous to ask. Nobody has minded yet, but some people might because I do have some religious content there. So far all the responses have been positive, and the ones I've contacted seemed to like being asked.
Donna