I let my big mowers (Kawi engines) idle for a couple of minutes before the first cut of the day or if they've been sitting for a while and have cooled off. My Honda 21 incher needs to be warmed up a good while before I can engage the blades or it will stall out, but it has the double blade system on it. I only idle them down if I've been working them hard and/or it's really hot out. Otherwise I just shut them off at idle.
For the 2 strokes I just fire them up and use them as soon as they will come to full speed and have good throttle response, which varies depending on the particular piece of equipment...my Shindaiwa handhelds and my Husqvarna trimmers are pretty much ready to work as soon as I start them. I just turn them off when I'm done with them.