Well, truth is, it's not so much the actual temperature, it's how FAST you can transfer that heat to the connection. The appropriate amount of heat really depends on how difficult something is to heat up. large or thick items tend to dissipate heat more easily, so you need more heat to deal with them, & for more delicate connections(like the little wires that go from the ESC & servo to the receiver), you need a little less temperature to keep from damaging the connectors or the wire insulation. If your iron has a good sized tip with alot of surface area, then you shouldn't have to worry about whether the iron's too hot, because with that surface area, you can heat things up fast enough to avoid damage(general rule, if you can do it in about 2 or 3 seconds, you should be ok)....