A timely thread. Especially since I came on to the forum to look into the same question.
In my case, I am planning to start mashing out due to the limits of my system. I'm a stovetop, batch-sparging brewer and collect my runnings in a big, food-grade bucket rather than into a kettle. I then transfer all of the collected wort into two 5 gal pots. The problem that is driving me to doing a mash out is that my beers have been finishing much lower than anticipated or desired (1.006 to 1.004). I'm assuming that letting the collected wort cool in the bucket without denaturing the enzymes is resulting in much more fermentable wort. My goal is to get my FG within the desired attenuation range based on my OG and chosen yeast.
I suppose that I could collect my first runnings and 'warm' them up on the stove, but ...
Thank you for listening to my tale of woe.