I know quite a few RTR manuals say to let them sit and idle. I never understood it either. Usually though, for the first tank or so, the fitment is so bad that they run like garbage anyway. Every engine I had so far just burps and farts for the first tank at least and never really gets to a high enough rpm to do anything useful.I don't know where that was said about just idol for break in.. you need to get the engine HOT.. so that the piston and sleeve.. can mold them selves to each other... sitting at a idol won't allow that to happen...
I tune them so they will run/idle and slowly accelerate from idle to WOT while driving it so there's load on it. I do that repeatedly throughout all the "break-in" tanks. As things wear in, I lean it a bit for each tank and usually after 3-4 tanks, they start actually opening up to a higher RPM, then I'm easy on the WOT part. Once the engines are tuned/running well enough to get up to 180F, I run them for minutes, not really tanks. I run them for 5 minutes at temp, ramping up to WOT in 2-3 second iterations, then shut them down after the 5 minutes, put the piston at BDC and let it cool to <100F, then do that for 4-5 cycles. Usually by the end of that, they are running more freely and I start driving them in grass or whatever for a couple tanks at a time while keeping an eye on temps and listening to how it's running and adjusting tune. After a couple tanks I'll put the piston at BDC and let it cool again.
Some engines really settle in and run well/respond to tune well after 1/2 a gallon. Others take 1.5 gallons+. Just depends on the engine and usually the build quality.