Allocate 20+% more memory that you could ever need. Use cgroups, lxc, systemd to constrain applications and containers to specific amounts of memory, cpu, etc. Properly engineered systems will have enough memory for everything else under the hood beyond the application. VM and container abstraction does not negate the requirement to calculate this.

Its a good question, but the swap advice is suspicious. If I would normally purchase a 4GB of ram instance with 4GB swap, then what happens when I purchase a 8GB instance? Do I still need 4GB swap? Hopefully this helps illustrate that it is really the applications you intend to run and the swap doesn't mean the system gets more efficient