Hm, that sounds a bit odd. If the idle drops or almost stalls out when you tip it on its side, that's often a sign of the saw getting too much fuel at idle - the excess can puddle in the bottom of the crankcase when sitting upright, and then drain into the transfers & cylinder when it's tipped on its side, flooding it. An air leak will generally cause the idle to race since leaks cause a lean condition, not rich. It is possible though; I definitely don't have enough experience to argue with a Stihl tech. I will disagree a bit with his point about not bothering to find the leak. It may be minor, but it could (and likely will) get worse eventually, and if you don't notice it in time it could roast your piston. You could at least replace the cheap and easy stuff like fuel/impulse lines and the carb gasket (though I can't remember if my 031 had one or not) since those solve most leak problems. You were also asking about a new rubber intake boot earlier, weren't you? Those can be problematic too, might want to check that out just in case.
