That said, I think the inexactness of that 1/30th-ish worth of 'overcharge' the first month will be negligible in comparison with others we can't avoid.  Our monthly charges will probably be a conservative calculation of the *average* cost of running a server for a month.

We will be managing proxies in different providers with fees and pricing models that vary between providers and across time within a given provider.  Digital Ocean has a monthly price that is the same in 31-day months as in 28-day months, but also an hourly price for instances that haven't run for a full month.  I think AWS charges purely by the hour (I'm ignoring reserved instances for the sake of argument).  Linode recently changed the specs of its offerings (if a provider we were using did the same, we could conceivably downgrade plans while maintaining adequate service).  I don't think we want to pass those details on to the users.  Arguably, that wouldn't even be fair unless we let them choose providers too.