It depends what kind of set your running..
As you know alot of people put protect on their pokemon if they have speed boost.
If you have protect, then just use modest, no need for more speed there.
If you are running a protect free set, you just need to know, that with modest, although not every opponent can outspeed you, there are people who still can without a speed boost.
Honestly , I think modest is the way to go, because after one turn, timid is pretty much useless.