I just link to whoever I find useful. Lots of sites have benefited from links I have given but have never borthered to return the favour.
Personally I don't guess either way, I link to so many sites on one page that each gains only a little. Except on my homepage of course, those are linked mostly internal.
My site is a information site, so the whole point is to help people go to the correct site. When I see people hitting my website with search terms that I don't have the answer to, I usually try within reason to create relevant links so the next hit with the same search term will be guided to the right site.
I hope to become a hub but I have no idea how you check if this is achieved. There are 2-3 other sides that probably are hubs in the same area and they have linked to me.
Very few people borther to link to me without prompting and they seldom tell me anyway when they do. But usually when I ask sites within the same topic to link to me the succuess rate is very high, but I havent borthered with a mass email to 100+ sites of similar content since each site is linked to another 2-3 common sites and eventually they will reach me too.
I only ask links for relatively high PR (4-5) sites and/or sites that might have visitors from different regions. Eg recently I got a university site based in Isreal to link to me. I can go all out and send a form email to the other small minor sites, but I figure it's not worth the effort and it makes me look like I'm all out to promote my site.
Oh yes, commercial links are great if you can get them...