We did it after, because we didn't know what dd was going to be, and wanted her to have 2 of her own sex and one of the other. We invited each one round when she was a couple of weeks old for an 'exclusive visit' and asked them then.
we asked before, with ds. and with this one havent asked the person i would like to be godmum yet, but dh has a close friend who once said "if you have another child can i be godfather?" and dh laughed and said "yes of course!" not thinking properly. so now he has assumed he is going to be but he is getting all up in my grill at the moment so really feel like telling him no. so this is probably why it is better to ask afterwards!