I tend to just top up by the exact amount I need for my combo ($46) just before it renews. I don't like having extra credit since I find accidental calls and stuff like that can use them up without my realising it. Having a zero balance at all times (apart from just before topping up) means every dollar I top up actually goes to something useful.
Good point about set-up and forget. It would be nice if it was possible for a monthly combo to be automatically renewed directly from a monthly credit card payment rather than having to go through the loops of topping up and then waiting for the renewal to be processed.
- James