I don't completely agree with the statement that using a buck/boost will only extend the battery life by a few hours. Using a coin cell that may be the case, but I will be using (preferrably) rechargable AAA batteries and my experience is that their voltage starts to drop very quickly, even if they still have lots of charge inside. So that's why I will be using a boost converter (boost only). I'll have to take care that voltage never excees 3.6 V and I believe using two AAAs that condition is met (as the boost converter can only increase voltage). If that appears a problem after all, I can try to connect both (or maybe three or four) batteries in parallel.
If I can't use rechargables after all, I could always opt for completely drowning normal batteries that have their voltage dropped too far for other uses. I've found out many appliances stop working even if there is lots of charge left, simply because the voltage drops. Some of them don't even work on rechargable batteries, for that reason. So I always have leftover batteries I can't use for anything else.