My understanding is that unless your system has an "urgent" feature that is high-energy-consumption and kicks in if there is a big difference between where you want the temperature to be, and where it is, it is *always* cheaper/uses less energy to limit the time you are trying to cool (or heat) your home.
I wouldn't consider a 1989 system "old," certainly not too old to have that feature, so maybe worth checking, but unless it's there, sure, I'd let the temp go up except when you (or DW) need it down to be comfortable.
My DH is very definitely not pregnant, but cannot sleep nights unless the temp in the house is no more than 76, maybe 74. Would that this were not so, but it is (unlike your HVAC, he is "older," so that may be a factor. Nice, as several commenters have suggested, to imagine that all our bodies work well forever but in fact, not all do, and even those that have, seem to do less so as we age).
Congratulations on the expected arrival!