If your vehicle has greaseable tie-rod ends, you can grease them as much as you want without hurting them, the grease will exit out the boot. (Although it will make a mess underneath) If you have ball joints with sealed boots, grease them only if needed. If the ball joint boot looks pregnant and is about ready to pop, don't grease it. A popped boot will only let in water and dirt. If you've got a vehicle with king pins, you really can't grease them too often. Just make sure all the pressure is off (wheels off the ground) of the ball joints or king pins when you grease them and if you've got king pins, rotate the wheels back and forth as much as you can to get the grease into as much of the king pin area as you can.
Of course, use an appropriate grease in the suspension area. A cheap grease won't do here.