I have been thinking a bit about ammunition. Stocking up on ammo for stuff you have is obviously the first thing to do. If you have a 12 gauge shotgun get some buckshot, stock .45 acp for your 1911 or whatever, you get the idea. Before trying to do anything fancy or creative buy plenty of ammo, whatever that means to you, for the guns you own. I have talked about the amounts of ammo that make me comfortable, yours may vary and that is reasonable.
Next I pay attention to the calibers that my family and friends own. Particularly the family and friends whose ammo cans are not exactly as stocked as they probably should be. For me the real common ground is .22, .38/.357mag and 12 gauge. I suspect .22 and 12 gauge would be good ones to stock deeper than you need for most folks. There are a lot of heirloom .22's and duck guns that only have a box or two of ammo to feed them. This is of course a much easier sell if the calibers they own are ones you also have weapons in.Talking to folks and helping them make good and common choices helps also here.
As I keep around enough .22 and 12 gauge for it to not be an issue the only one I have had to pay some extra attention to is .38 special. I stock it deeper than .357 because it is cheaper and also because .38's will go into a .357mag but not visa versa.
Some folks suggest stocking common calibers in case you need to resupply a friend or trade or whatever. I wouldn't put a lot of money into calibers I do not have weapons in but can see some merit in the idea. If buying a couple hundred rounds or a half case of 7.62x39 or .308 or whatever common caliber(s) you do not own makes you feel good that seems fine. Personally when I wish I had kept 250 rounds and a couple mags when I sold the Glock .40. It is a common round and I have 3-4 friends with them so it wouldn't be bad to have just in case. If/when I sell the 1911 I will keep some ammo and a couple mags though I do not see myself getting back into the platform/ caliber.
I once heard on Survivalblog that it is a good idea to also get a few boxes each of the most common 4-5 hunting calibers in your area. Of course assuming all your other ducks are in a row buying a few boxes each of 30-30 Winchester, 30'06, .270, 7mm mag and 300 Win mag (that was just me guessing but I bet it is close) might not be a bad idea. I suspect a lot of hunters are in the one box of ammo club. Personally I would like my well intentioned neighbor's rifle to be loaded!
Anyway those are my thoughts on that.