I would do a bit of research first.
Lower Resist drops off steeply, so I generally put 1 point into it and that's it. Check Arreat Summit for the numbers --- they very much encourage the "leave it at one point and let +skills raise it a bit".
*HOWEVER*, if I were to make a poison necro, things would a bit different: 1% difference can mean the difference between breaking poison immunity or not breaking it. A 50% Lower Resist can break immunes with 109% resistance or less, a 55% Lower Resist can break up to 110%, a 60% LR an break 111% and so on. So it could be a good idea to check some monster data tables to see whether, for example, there are 111% immunes --- because if there are, taking LR up to 60% is a good thing to do, even if it means investing more hardpoints into LR.
(Note that I don't know the skill values, so the 50-60% range I'm using is just an example).