Steeping is kind of an imprecise science. There are a lot of people that have a lot of beliefs about what works best but as long as you keep the temp between 150 and 170 for half an hour you are doing it right. In fact I've seen people simply steep till it gets to 170 however short that may be. I wouldn't suggest any less than 20 minutes and I think you are better off waiting the whole half hour, you get more flavor out of your grains that way.
I use steeping opportunity to practice temp control for when I finally go all grain so I try to stick to 150 for 20-25 minutes and then crank up the heat and take the steep grains out before it hits 170. I've never tried steeping when the water is cold, I don't think you'll extract much at that temp, but I can't imagine it would hurt anything, but someone more knowledgeable correct me if I'm wrong.