Here's my take on a good leveling spec:
Level 11:
http://db.vanillagaming.org/?talent#fMRemorseless Attacks helped me a lot to kill mobs faster. Kind of difficult to utilize this if you go for mobs the same level as you since you might need to heal in between everyone (early level).
Level 19:
http://db.vanillagaming.org/?talent#fMZGcGetting to backstab crit asap.
Imp gouge: gouge + bandage has saved me many times from dying while pulling more than 1 mobs, or 1 hard one.
Imp SS: somtimes you want to fire off an attack to kill off a mob at the end, and now you can for 40 energy instead of 45.
Level 37:
http://db.vanillagaming.org/?talent#fMZGcZE0bccGetting to imp Sap asap while getting damage boosters and making it easier to stealth. Sapping and getting back to stealth helps a lot when going after more than 1 mob at a time and the improved stealth helps from not getting detected. You might want to change how many points you put in deception/camouflage.
Level 50:
http://db.vanillagaming.org/?talent#fMZGcVb0xzZE0bccGetting to Blade Flurry asap which again helps out at more than one enemey while getting more damage boosters (+hit and +crit).
Level 60:
http://db.vanillagaming.org/?talent#fMxZGcVb0vz0oZE0bccFinishing off with +5% crit in assassination and offhand damage + inc weapon skill in combat. In that order.
The focus in this build is backstab, ambush, stealth and sap. You want to maximize backstab damage, and critrate, while minimizing downtime between pulls.
As OP said, you can always use backstab by sidestepping in mobs and spamming backstab until you get one off. Quite easy with a little bit of practice!