Which Hecatomb is skippable?

Level 50 Inv/SS tank. I currently have all my melee attacks six-slotted with Crushing Impact. I plan to begin gradually replacing the CI's with Hecatomb's. I'm wondering if it is feasible to only 5-slot them and am wondering which Hecatomb to skip?

Depends on your build. For some it might be Damage, for others it might be Acc/Rech or Acc/Dam/Rech; I don't think I'd ever skip the proc (except for Ragnarok's) unless I had to.

Edit: an example of a time when not to skip the Damage IO is when you already have high global accuracy and the power in question already has the recharge it needs to fit into your chain seamlessly. In this case skip the Acc/Rech and get the extra 10% damage from the Damage IO.

Skip the dmg. Assuming you use the other 5 for the recharge bonus, it'll put you about at the ED cap

Skip the damage IO.

I take one of several approaches. I don't usually like to be below 95-100% slotting for the primary attribute of my powers (so damage for attacks). Skipping the straight damage IO will leave you at around 90%.

If I want good recharge slotting and the power itself has large base damage and a large base recharge time, I'll skip the proc. That's for PvE, where the proc is primarily used to increase DPS. In PvP, you might want the proc on a large scale attack to try and deliver better spike damage.

If I want the proc (usually in a small-to-mid scale attack with fast base recharge) I skip the damage, but usually add another "frankenslotted" sixth slot to the attack. I usually pick an dam/end if I can get it, since this maxes out my ED damage and improves on something purples leave a bit to be desired on compared to a lot of more mundane attack sets - end reduction.

If I want either to use just 5 slots in the attack, or to put something completely unrelated to damage in the 6th slot (such as an unrelated proc or even a generic level 45-50 end reducer) I usually skip the damage/recharge. I tend to do this in attacks that already have low base recharge and when I am going to have good global recharge bonuses. Unless you have super tightly tuned attack chains, you probably won't notice the difference on anything with a 4-6s base recharge time, especially if you end up with 40-60% or more global recharge.

My general rule when slotting purples (not just heca's):

If it's single target, I skip the proc and if it's an AoE I skip the DAM because AoE's usually recharge slower and benefits more of cutting a third of the recharge time than adding 5% more damage, and the proc has better chance of firing against multiple targets.

It depends a bit on which power you're slotting it in. If you're putting it in Knockout Blow (the power I'd but it in) then I'd probably skip either the Damage or the Proc. Skipping the Dam/End is the best choice if you can handle the increased endurance costs but it's an expensive power so probably not a good choice. With the Damage or the Proc you're decreasing the average damage you do so it's more a case of do you prefer consistent damage that is enhanced by damage buffs or erratic, unenhacneable but higher damage. You could also skip the Dam/Rech depending on how solid the rest of your attack chain is.

If you're slotting it in one of the other SS single target attacks then skip the Dam/Rec. The Damage loss is unnoticeable and you don't need the extra recharge.

Do you intend to exemp a lot, particulary very low ?

Going over the cap can be useful in that one circumstance.

Why, well say you run a posi.

You exemp from 50 to 15, and using this table (culled from the wiki):

1  0.022     17  0.454  
2  0.045     18  0.486  
3  0.068     19  0.519  
4  0.092     20  0.553  
5  0.116     21  0.587  
6  0.141     22  0.623  
7  0.166     23  0.659  
8  0.192     24  0.696  
9  0.219     25  0.733  
10  0.246     26  0.772  
11  0.274     27  0.811  
12  0.302     28  0.852  
13  0.331     29  0.893  
14  0.361     30  0.935  
15  0.391     31  0.978  
16  0.422     32-50  1.0

In almost all 5 slotted purple sets I have skipped the acc/rech.

The exception being Ragnorak which was in shockwave, and thus the knockdown proc was superfluous.