Well, in the first place I definitely won't buy ammo unless I already own something to shoot it in.
Since about half the rifles in my safes take oddball cartridges, once I acquire a rifle the next purchase will likely be dies and brass instead of loaded ammo, assuming I don't already load for it. As a general rule I like to have between 50-100 cases on hand for every rifle I own.
If a rifle I want is available at a good price and cases aren't, I'll buy the rifle and wait for the brass suppliers to catch up. I keep enough powder, bullets and primers stocked up to survive ammo droughts, so timing purchases for those items hasn't been much of an issue.
There are a few cartridges I'd rather buy than handload, however: 9x19, M80 7.62, .223/5.56, .32 ACP and of course the rimfires. I did defer buying new firearms for a couple months in favor of stocking up M80 ball ammo two years back, and boy did that seem like the right move in hindsight! Between the Obama drought and the current situation I stockpiled 14K rounds of .22 LR, but I did that at sale prices over the course of about three years. And right now my current stock (about 900 rounds) of 9x19 is lower than I'd like, so I will either replenish at the next opportunity (and for less than an insane price) or have to add it to my handloading rota.
(BTW these are samples, not the whole works)
Buying guns for me is about interest, availability and price, and that holds true at any time. I guess ammo is more about price.