I know the answer to your question Cachergirl so all I can say is that I left the seed heads on my Allium Christophii last year and they are not very good this summer. Whether that's the drought or whether I should have cut them back, I don't know.

Feed them rather than cut back. If they are species like christophii then the seed will come true. The reason some advocate cutting back is because the bulb is now putting energy into the seedheads rather than next years growth. Personally I don't dead head anything until it is really looking untidy and I feel that I have to!

Actually with those big Alliums, the seed head and stem is already virtually disconnected from the bulb by the time the flowers have finished, so there is probably little point in removing them. I never bother and we have a large number of self seed ones all over the garden. The time to feed these Alliums, by the way, is when the leaves first appear.

I agree dead heading will only prevent self seeding but the leaves that are needed to build up the new bulb have gone so no point in feeding now.
my A.christophii are just as big this year as last.

They are good in large dried arrangements over the winter. You can spray them gold if you like.