"In my opinion you should never 'squeeze' it tighter than the clip / fastener is going to hold it. Otherwise you squeeze too much out and are left with voids. "

Disagree. What seem like voids are either the points of actual metal to metal contact or so close that you can't see the compound. Remember I said a bead or daub, if it is spread it is almost for sure not even and that will cause actual voids where it traps the air at the low spots of the compound. Some of those clips are too weak to overcome the viscosity of the compound, and some of that aluminum stuff they make the heatsinks out of is very soft and the screws might strip. The main thing is not to let it come up or move. If it is done right they are usually stuck pretty good though.

"I've done a lot of work on laptops, a lot of them nasty to get into so it's best to do it right first time."

Agreed, with anything. It's been said "How come there is never enough time to do it right but always enough time to to it again". I have even told people now and then "Hell no I don't want to do it right, I just don't want to do it again". I also don't want the liability, if the part fails guess what ?

"I've also lapped more than my share of CPU
'spreaders' (IHSs) and (desktop machine) heatsink bases. "

I haven't done that many processors in PCs, I just follow instructions. I did much work in power supplies and amps, unfortunately lapping is simply impossible on most. I also agree with Jeff Leiberman's caveat about making the surface concave. If you do that it could be worse than not lapping at all.. I only lapped in some output ICs once actually. I was extremely low on compound so I thinned out what was leftover and lapped with it. The ICs required no insulator being a totally plastic package. At the end of the lapping I had them in position and moved no more so that if there were any larger particles in there they would stay put, embedded and not keep the surfaces away. The only reason I was even able was because I was retrofitting a pair of LM3886 in place of a big STK and made a clip that went from one mounting screw to the other. I had plenty of room. (I would have put i pre outs and had the guy use an external power amp but the damn tone controls were in the global feedback loop)

Luckily I don't have to do much of that anymore or I would have a caulking gun full of compound like this one place I worked. Makes it easier to apply and I would never run out. They did but it took years even with a bunch of techs working.

I've been hearing of laptops with mo fan, and of course smartphones don't have them. Will it eventually get to the point where they don't need heatsinks ? Get an efficient enough switch and it could happen. I have already seen power switching transistors with no heat sink except the tab itself.

At 0 Vcesat, 0 Vceo, 0 Vbe, 0 tOFF and 0 tON there is no heat to sink.