These days, I try to stick to around 40 pounds for a longer trip. Seems like (a) everybody's more strict about weight limits these days and (b) I always seem to have at least one check-in along the way (on longer trips) on a 20kg airline.

I've flown PNR's where I've printed the detailed fare rules and found ambiguous wording as to what baggage allowance applies across that ticket number. I always assume worst case: the lowest allowance applies to the whole itin. (Or, in practice, the lowest allowance where you're actually performing a check-in with the bag.)