Just to add to the issue, there is a potential problem when mounting steel rings to an aluminum base. Specifically, I have a SCAR-17s with an integral aluminum rail machined into the upper. I had steel Mark 4 rings from Leupold for my Mark 4 scope. I noticed that when I took it off there was a visible marking on the rail. I called Leupold and they emphasized that they HIGHLY recommend using aluminum rings on aluminum rails. They explained that going the other way was not a problem but, even following their torque recommendations there is a possibility of deforming the softer aluminum by using steel rings. So, I picked up an aluminum base with 20 moa from Aadland and the problem was solved (as well as my lack of elevation). I examined the rail with a 10x magnifying glass and it was clear where the steel rings had started to distort the aluminum base. I then called FN-USA to ask their opinion. They explained that the rails are hardened and they did not expect much damage from using the steel rings. When I explained that I could see changes in the edges of the rail they explained that the indexing is done against the raised section of the cuts and that should not change. I am not sure what is best for general use, but I figure that my $3000 SCAR is worth protecting as much as my scope. Since I don't want distortions or scope rings on my scope I also don't want dings on my rail. From now on I will follow Leupold's suggestion to use aluminum on aluminum and use anything you want on steel. So, mounting to a steel rail aluminum is fine. Mounting to an aluminum rail with steel will probably not affect performance but will affect the look of the rail. If you ever want to resell it, having it look good will have a lot to do with the resale price. I recommend aluminum on aluminum and anything you want on steel.
The key thing to remember is what is going on at the hottest interface. On a bolt action rifle with a steel receiver, a one piece rail is pretty straight and stiff in the axial direction. So if you use an aluminum rail on a steel receiver and fit it at 75F and then go hunting at 10F the rail would have shrunk a lot more than the receiver ans this can set up stresses to the point where the rail could be deflected, even if this is elastic. This would shift the point of impact.
Take the same rifle shooting prairie dogs and the barrel could get pretty hot, now the rail would expand more than the barrel and deflect the opposite way.
Where the rings attach to the scope base, they are much closer together than the rail overall length, so the total dimensional change is less. Consequently, even though the scope tube is aluminum, with the rings being close together less deflection will be taking place and at some point a change in impact would not be measurable.
I just make sure to match the scope base material to the receiver and loctite it in place. I would agree to be careful to use steel rings on aluminum, the steel will win every time. I don't see any reason to use rings strong enough to lift an SUV on a battle rifle. There has to be a weaker component in the scope or the scope tube will simply crush before the rings let go, so what is the point ?
Most of my rings are located right over the top of where the scope rail screws to the receiver. So the distance between the scope rings on the scope tube is the same as the distance on the scope rail, which is where the scope rail is fastened to the receiver. Differential expansion or contraction of the receiver will only act differentially on the scope rail where it's bolted to the receiver. They're all the same distance apart, and any differential expansion or contraction will be acting over the same distance.
The distance that the scope rail extends past the scope base screws, or the scope bases, doesn't even come into play.
Other than concern about the cosmetics of steel leaving a mark on aluminum, the rest of this discussion is largely witchcraft.