Hit => Expertise>
Keep in mind some of these change depending on caps or just pure amount you can get. First off Hit and Exp obviously lose any value after 7.5% However Hit and Expertise can be more worthwhile than haste if you just cannot get a decent amount while maintaining atleast close to cap on both hit and expertise, it is likely safer in normal modes and early heroic tiers to maintain hit @ cap and expertise you can be a little more iffy about in favor of more haste. Crit even w/ all of blizzards changes is still not worth trying to attain.
Tier 1 Nightstalker Subterfuge Shadow Focus - Optimally in a patchwerk fight subterfuge is superior, a bleeding ambush into a rupture is pretty hard to pass up. However the easiest choice would be Shadow Focus this is really just going to make energy management easier at the opening and vanishing points.
Tier 2 Irrelevant for DPS, unless your forced to range DPS then clearly deadly throw is a dps increase.
Tier 3 Again no "true" dps increase, procs can occur from Leeching poison but its your most obvious choice in pve anyways.
Tier 4 Preparation Shadow Step Burst of speed - Raw DPS goes to preparation, Shadow Step mobility will usually win out, and burst of speed can be a nice raid utility.
Tier 5 No dps increases to be found.
Tier 6 Assassination hands down anticipation even in Heavy switching fights your ramp up time is not long enough to pass up being able to manage rupture and get as many envenoms out as possible w/ horrible resource regeneration currently.
This has honestly become the worst possible addition to the game and very trivial, Vendetta can be a dps increase if you can get full duration 100% uptime.
Opener[Shadow Focus] - Prepot stealth, Shadowstep unless something is coming quick you may need it for, and begin your damage with a garrote, this will allow you to open right away w/ a bleed and a 1 or 2 pt Slice and Dice for going rupture.(If preparation specced donot rupture until you have vanished and your 2nd garrote is running out.)
Opener[Subterfuge] - Prepot stealth, this go round you will open w/ an ambush then go directly into a garrote, mutilate once to get a 5+ pt envenom off then SnD asap afterwards.
p.s. in both cases you want to use Shadow Blades and Vendetta before you begin any part of an unstealthed DPS rotation.
Filler - Mutilate and Dispatch procs will be the main fill of your "ACTIVE" dps. If you do not have a garrote or rupture present or running dangerously low Rupture will be your goto finisher(alternatively if you have Vanish off CD use garrote). You will never apply a Rupture or Envenom w/o having 5pts, unless Rupture or Slice and Dice are dangerously low on duration, however unless you have weird offtime you shouldnt have trouble maintain both of these.
Dispatch Pre Execute - Using dispatch on proc and holding it can both be DPS increases, you will need to know when you need to sit on it or not. If it procs on a mutilate and you already have 5pts just Envenom even if its going to heavily clip your Envenom buff, using it before a finisher or directly after while having medium or high energy is going to possibly cause energy capping this is severely bad in assassination. Now it is also wise to sit on a dispatch if for w/e reason you need to get SnD back up (i.e you have CD's and Pot's running with 1-4 combo points w/ 1-3 seconds left on SnD, you should not reapply SnD w/ a 5< Ev or a 3+ SnD it is more ideal to continue with your big hits and manage your energy.)
Dispatch Execute - This will mostly replace your mutilate during execute, except when a good crit string and or finisher string puts you at high enough energy to use mutilate @ which point to prevent any possibility of energy capping dumping a mutilate is going to be a better idea. During this phase Envenom will begin to teeter above keeping rupturing rolling, so you will want to keep an eye on the boss health as putting a full duration rupture up @ 10 seconds of boss health is going to net you lower numbers, if your stuck w/o energy it is not a bad idea if you can gauge it well to apply a rupture w/ a shortened duration Venomous wounds is just too nice to pass up. Shadow Blades is best used in this phase so holding your cool down for a SHORT time rather than using it on CD can be a DPS increase.
AOE Simply Fan of Knives til 5 combo points and envenom, keep envenom rolling and keep using FOK. Fan does not suffer from AOE cap, and in assassination spec the instant poison applications of deadly poison are just better than Crimson Tempest.
This concludes My shorthanded and quick Assassination guide.