Stalker power selection is very tight, at least in the early to mid game. AS coming available at 6 - you can't pass that up, let's be honest. Placate at 12 - far too useful, one of the iconic Stalker powers (classic 3-mob engagement strategy is AS #1, scrap #1, placate/AS #3). So at either 8 or 10 you need to pick up the prereq for your chosen travel power. I don't know about the other sets, but Build Up comes in there for me as well (Ninja Blade) - it's not mandatory, but imo you'd be crazy not to take it.
Assuming you pick your second attack at 2 and a defence at 4, that's your power build spoken for up to 14. Regen and SR will have 16 already mapped out to Integration or PB. As a SR Stalker, I'm guessing you - like SR scrappers - will want Stamina asap, so yes, 18-22 are also covered.
I can't think of another AT whose power picks are so heavily... well, almost predefined, really, for so long (I'm sure someone will come along and prove me wrong, but still).
But it sure is fun watching the enemy uttering dire threats as they collapse to the ground... 
I got my powers in this order
Lunge, Hide, Barb Swipe, Reconstruction, Assassin's Impaler, Hasten, Swift, Placate, Superspeed, Integration, Health, Stamina, Impale.
I believe I went for the minimal secondary choices and even then I had my first real choice at level 22! The next decision ( at level 26, build up is at lvl 24 ) will be a hard one as I have to decide between Ripper or a defensive power, like Dull Pain.
In the run up to 32 I have the following 'must have' powers to pick, Instant Healing, Ripper, Dull Pain, Build Up. All of which need between 3 and 6 slots.
So if you want Stamina, which you will need if you want to do any scrapping with anything but the first 2 primary powers, you have little chocie at all.
@Unthing ... Mostly on Union.
My stalker is currently lvl 12 and is ninja blade with ninjitsu sets. And following was my flow of choices:
at creation: Sting of Wasp/Hide
lvl 2: Danger Sense
lvl 4: The other ninjitsu DEF buff
lvl 6: Assassin's Blade
lvl 8: Build up
lvl 10: Flashing Steel (just to get a second attack)
lvl 12: Placate
The only thing that I perhaps could have switched with something else is the Flahsing Steel, with a requirement of travelform.
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I'm guessing 12 is Placate, 14 is SJ, 16 is Practised Brawler and then 18-22 the fitness pool.
Which doesn't leave much more space for secondaries.
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What youve written sounds pretty good to me. The only secondary you might want now is hasten but as youve gone for SJ i shouldnt worry about it til later. The only thing you could do is postpone Stamina til lvl 24 but youll probably want focus ASAP.
Personally i wouldnt have taken your earlier choices. Id have taken Swipe, strike and focus fighting and not the other attack or focused senses. But your choices are still good and your build strong.
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Is placate something I can leave til later?
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No i dont think so it really is a must. Ive slotted 3 recharge solos and run hasten as much as possible and it still doesnt recharge fast enough for my liking. Its tricky to use tho and takes a bit of practice. For instance you can AS a foe even if your not "hidden" as long as he cant see you and you dont get hit.
Heres my NB/NJ build so far if your interested:
0: Sting of the wasp , Hide
2: Gamblers cut ( unconventional i know but it suits the stalkers "1 on 1" feel and it has higher accuracy than the others. )
4: Danger sense ( I prefer ranged and AOE defence as this keeps my hide on and allows me to run away safely )
6: Assasin's blade
8: Build up ( this is awesome, its a must pick )
10: Hasten
12: Placate
14: Super speed
16: Kuji-in Rin (mez protection like PB)
18: Swift
20: Health
22: Stamina
24: Ninja Reflexes (better late than never - if i got attacked in combat i just ran anyway til now )
26: Soaring Dragonfly
Im very happy with my char at the moment, especially in PVP. My very high AOE defence means i can get close to blasters/controllers/defenders easily.
Then Build up + soaring dragonfly + placate (whilst there still in the air from the knockup!) + AS will kill any of them no problem. Then i use SS to get away fast!
Hope this helped 
what excatly does Placate do? Havent checked out that power much?
Placate makes one foe stop attacking you and putting you back in a brief hidden form. It only works on one foe though so if you are up to a boss, build up - AS, fight a few moments until AS is almost reset, then placate and AS again. With a few recharge enchants in AS it works pretty fine, two AS in one fight before you have to run
very nice.. thanks for that
Just wondering how people have managed to fit in their powers?
I'm claws/SR (stop laughing!) and am currently level 11. I've got two powers from my secondary (focused fighting and senses) 3 attacks (including AS) and Combat Jumping.
I'm guessing 12 is Placate, 14 is SJ, 16 is Practised Brawler and then 18-22 the fitness pool.
Which doesn't leave much more space for secondaries.
Is placate something I can leave til later?