IMHO, there's not point investing more than 40$ in a CPU, hence the celerons work just fine. On my rigs, CPU is barely used.
Also installing i7s and the like will just up the wattage (even if it's just 10-15 Watts).
Unless you plan using the hardware for other things, just get the cheapest Celerons, they'll work just fine for mining. Also, new coins usually drop CPU mining rather quickly anyway so I don't get the point of purchasing CPUs that cost as much as a good PSU, which is far more important.
Just my 2cts, I guess everybody's got their preference