In real world performance, no you wont really notice it. Usually only benchmarks and hardcore memory overclockers can notice differences. You can leave it at 2.5-3-3-5 and be fine. What type of memory are you using? If your using high end stuff like OCZ gold or platinum or Corsair XMS, you can set the timings lower like at 2-2-2-4, but don't recommend it unless you're sure of what you're doing.
EVGA 680i SLi
E6400 @ 3.0 GHz
4 GB OCZ Platinum Rev. 2 EL @ 880
400GB + 250GB Seagate Barracuda SATA2
Antec P180B Case