i know this is not for me but here is my $0.02.
I personally wait untill you get the email and respond to if becasue the offer call still needs approved by HR thus you get the offer email later to respond to.
I was told during my offer call that all the paperwork would need to be approved by HR and i will get the offer e-mail with in a week as long as everything was approved.
Hope that clears that up for you.
I start 4/1 and currently have a job.
i got offer call and email.
I could work all the way until right before i start if i want to but im actually putting in my 2 weeks in, in about a week and a half

(cant wait)
So i'll have a month to relax and set up my office!!
Im also going on 2 much needed vacations 1 with my lady and the other with my older brother.