I have just started triathlon recently and have been wearing socks in the 2 races I have done. I have heard that pro's don't wear socks.
Is this true? I have done a 5km run without socks but I feet that that was the limit. Any more and I would have blisterred.
Shoe type and shoe fit are a great start. Lubing either your feet or the inside of your shoes helps too. A trick I like and use is liquid bandaid on the "hot spots". It forms and extra layer of skin and works great.
Personally my cutoff is 10K though. Anything over that and I sock 'em. Good luck.