Depends on the job really. If its a big job and I know they are going to be looking at other quotes then I would normally post it, as I know they are going to need a bit of time to think about things.
If its a small job and I can pretty much walk in and know they are gunna accept it, I'll either go back and see them with the price or I've found even better, pop in the van for 5mins work it out and go straight to them with it, much higher chance of them accepting especially if you have gained thier confidence while in the house having a look around and a chat. (although at first maybe not the best idea as your always gunna forget to add something)
You'll learn to read clients well and know if they are fussy or couldnt give a monkeys and just want it done.
Nothing wrong with calling them though, but I find giving them something in thier hand to ponder over, looking professional and so on is slightly better than a call when they could just happen to be in a crap mood and its not a good time.