I like one-piece aluminum bases because they notably reduce weight. I know how to calculate the differences in expansion between steel and aluminum over the approximately 5 inch spacing between the forward-most scope base screw and rearward-most base screw (which is the effective distance the two dissimilar metals would bind over), however I have never done it. I just don't expect the difference in the rate of expansion over this 5 inch distance would cause problems.
The part that's always bothered me about the concern that aluminum bases would cause a problem on a steel receiver, is that the scope tubes are also aluminum. So even if we use a steel one-piece base on a steel receiver, we're connecting them to an aluminum tubed scope that will contract and expand at a different rate than the steel receiver and steel scope base. So the same concern would apply - that the differential expansion & contraction of the scope tube versus the steel base could cause some type of problem. Yet Aluminum scopes are the norm, and Steel receivers are the norm.
So I don't personally give it any further thought. I do go with one-piece aluminum bases to get 4 scope base screws holding a single scope base, rather than two scope base screws holding two different scope bases. Four screws holding one base over a length of 5 inches is simply a more rugged mechanical connection, less apt to shift position during the thumps and bumps of the hunt.
Just my thoughts on the subject.