I like to dry-hop in primary after 7-10 days or so when my activity slows down considerably. As long as I don't leave it sitting on the yeast cake for 3+ weeks, I think it is okay to dry-hop in primary. I think my hops get absorbed into the beer better when I can loosely throw them in (more surface area exposure) as opposed to dry-hopping in a mesh/cloth bag in secondary. I've always done it this way and my LHBS think my beers that I dry hop have an incredible nose to them. I'm sure others will disagree, but from my experience I like it this way much better.
I usually ferment @ mid 60s for ales for about 14 days. I pitch lots of yeast so it finishes out quickly. Then I warm up all my beers to 70 degrees plus to finish fermentation for a few days/remove DMS. I cold crash to low 50s or so (depends on what my garage temp gets depending on the season) to clear the beer. This has been very effective for me. I personally would not want to "shock" my ale yeast by lowering it to near freezing temps. It's probably not a big deal to some, but I get very clear and clean beer by lowering it to 50s/keeping it very still before siphoning.