I don't play CoD, but when I play CS:S it depends on the map.
Minigame (sniper): Stay back while moving forward ever so slowly
Minigame (assault): Run in, nade, get a few kills, die
Minigame (Pistol): Stay back, headshot from far away
Minigame (knife): I knife people.
De_dust2 (sniper): I stay at T spawn for about 20 seconds, then go to B and use my deagle to camp there, use sniper to shoot people down to CT spawn
De_dust2 (assault): I rush to A, protect that and shoot people half the ramp up to bomb site B.
Other maps: I just use whatever seems best at the time.