Well... for me, it's all about comfort and what I can physically do without hurting myself.
From the swim to the bike... I sat down... wiped my feet off (as I had to run through gravel with wet feet)... and then I put on my socks and then my shoes.
I ran in my shoes to the line where I could get on the bike... and away I went. Remember, if you put the shoes on the pedals... you will have to walk in your socks out/in of the transition area.
Then, when I got back in... I just took the shoes off and put on my running shoes... away I went.
Mind you, I am NOT a speed demon in the transition areas... then again, I am not trying to be in first place... I just want to have fun and finish.
Practice before the tri and see what works best for you in your transitions.
I know for me, I don't think I would not be able to get into my shoes already attached to the bike... and I don't want to run in my socks and get stuff stuck to the bottom, that's going to be in my biking shoes... and then in my running shoes. That is not comfortable to me.