They do seem to halve the price of some titles with pre-orders atm too. Ultimately I guess it's how up to date you wanna be and in at the thing, but there's no reason why waiting for discounts or sales or trades. I normally do it that way, but I do want Marvel to know I like the idea of Secret Wars by getting it at the time.
Comixology is incidentally having a sale of Hickman's Avengers run until the 7th, if you don't have any you can get all of it for £50 (or whatever that is in dollars) with the two bundles (which have all the Time Runs Out issues as exclusives).