Leg sweep should be used on any fight that does not require stun/interrupts (you would affect diminishing returns, and could make the target immune)

Leg Sweep is off of the gobal cool down. It is used between normal abilities without slowing down your rotation or affecting combo strings. It is completely free extra dps, and this should be mixed in on cooldown.

Feint is not off the global cooldown and results in a dps loss... The Debuff it provides will slow the targets attack speed. It can be useful to reduce damage taken. Most bosses that do enough damage for this to be useful are actually imune, so it will not be used much in the endgame.