hey it's cool. We all have different bits of knowledge, and we're bringing it all together here. You were given good advice, to make sure the voltage is the same, and the same polarity. That is the right rule of thumb. But in this case, with a voltage like 19V, it is safe to go a volt either side. At that voltage, it won't make much difference to the current running through the device. Using 6V on a 5V device is a whole other matter, where the tolerances are tighter. Same for CPU's, where 0.25V can make the difference between working, and magic smoke. It's not always safe to go lower in voltage as in some circumstances it can increase the overall current, as efficiency drops, especially for DC-DC converters. Have you ever looked at PC PSU tests which show the efficiency curves at 110V and 220V? Ever noticed the supplies run slightly more efficently on 220V (240V here in the UK)?? It really depends on the device you are supplying whether going up or down is the best idea. Also if you are near a capacitor standard voltage setting, it IS sometimes better to go down rather than up. they tend to be 6.3V, 10V, 16V, 25V, 35V and then your into the bigger stuff. In small circuits you usually never go above 35V. Anything like a laptop input, I would expect a 35V cap, but it wouldn't surprise me if some manufacturers chose the 25V one to reduce size. That's where the danger can lie, when you get close to the rated voltage.
Also you are right that most laptop PSUs are considered to be 90% efficient, but TBH that probably their peak efficiency at a medium load. We don't really test them like we do PC PSUs, so who knows? Maybe a test we should do, as see what the reality is. The thing is, at such a low wattage, then tend not to bother, as you are only talking a few watts here and there. 80% of lappys run on either 65W or 90W PSUs. Only the gamer ones which we are more interested in will use beefier ones. So in most cases your arguing over 7W-10W which is hardly worth the difference, unless you have many multiples of supplies.
As for power jacks, I've replaced a fair few in my time too. Again, a higher voltage could actually mitigate this. Really they are shoving too much current through those jacks though, and most I've replaced have had rather anaemic cables or track pads on them. Upping voltage, can increase inrush currents at the start, lowering voltage can increase continuous currents as the DC-DC converter switches on for longer to compensate. so it can be hard to figure out which way to go if you don't get an exact value. the thing that melts jacks though, are increased currents, so I would actually tend to head up in voltage a little, because overall you reduce the current a little. The hardest part for most folk to get their head around, is it's the current that does the damage, the voltage is just the driving force. And in pure resistive loads, you increase the voltage, you increase the current. It's a whole different ball game though in active circuits, like DC-DC converters. That's why I laugh sometimes at the arguments here as to whether smoothing caps have current flowing in them or not. Caps can have a LOT of current flying through them, yet folk don't think they do if they are acting as a smoothing cap. I dread to think what flies though some of the motherboard caps, where there could be 100+ Amps flying through the processor at full load.
Anyway don't sweat it. It's good to have debate, that's how we all learn. I've learned LOADS from others here. Even with my HNC in Electrical and Electronic eng, I don't know everything.