Venom Grenade is not auto-hit so it's worth slotting in a bit of accuracy. As for why you are rarely seeing it miss I suspect your build includes both Tactical Training: Leadership and Combat Training: Offensive. If you have both of those powers and have a little bit of slotting in them then you will get a 95% to hit change on all of your powers against even level enemies. In general it's worth adding a bit of accuracy slotting to help out in situations where you need more either due to higher level enemies, defense buffs or to hit debuffs.
Thank you very much! That does make a lot of sense. And yes, I do have those powers. Guess that's why I rarely see a "miss" In the future, I will add 2 accuracies and 2 recharges. Thanks again!! 
If you have those two powers, I'd do 1 Acc/3 Recharge
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My crab spider just hit 28. I chose venom grenade. A pal of mine suggested this power to me because he feels that its a great debuffer. That's why I picked it. I noticed that I can add accuracy enhancements to it. Here is my question: is it really necessary to additional accuracy to this power? I don't think so because every time I use it, it looks to me that it hits the enemies all the time. I never saw a "miss" when I use this power. Even on bosses. Someone please clarify! Thanks!