I personally wouldn't use daggers to level until you have enough talent points to get opportunity and seal fate (so that would be around 45 ish). It's just slow and sucky before that unless you are using sinister strike as your primary attack - in which case, you should be using whatever weapon you can find that has the highest average damage range (in your main hand). Generally, this is going to be a mace or a sword.
Daggers really don't come into their own until you get a bit higher. I leveled 60-70 as mute, and it was a blast (and went very fast). However, it's pretty crappy before you get your stuns (CS and KS are both late 20s-early 30s IIRC). Honestly, it's pretty crappy without seal fate in general.
There is a damn good reason that pretty much every combat dagger rogue would have a sword in their bags to farm with (at 60 when you saw a lot of combat dagger rogues). It was great for raid damage when you could stay behind the target. However, it was poor for farming/grinding (which is pretty much what you do when you quest/level) because it wasn't energy efficient to stay behind your target.
If you want to level with daggers, by all means... go for it. However, you aren't going to find an efficient low level dagger build for solo PvE leveling/grinding. It just doesn't exist. Using gouge, to wait for a backstab that will only give you 1 CP isn't efficient. Even after you get your stuns, it's lacking. Once you get 30 points in assassination + 5 for opportunity, it's actually not that bad (the old PvP SF dagger builds were pretty good for farming/grinding). Once you get mutilate it gets better still.
The problem is compounded by the fact that there aren't as many good dagger choices as there are sword/mace choices while leveling. There are nice sword quest rewards from both SM and RFD... then when you can do a princess run, you can get a thrash blade that will take you up into the upper 50s (where you can grab an outland green). Before that, there are swords (and a dagger) from dead mines, a nice mace from WC, etc...
The bottom line: It takes 2 talent points to make a viable/efficient sword/mace leveling build (imp SS 2/2). It takes far more points to make daggers efficient for solo content. If you had a warrior/pally/druid leveling buddy or a leveling group of friends, my answer might be different... but for solo work... you'll level significantly slower if you use backstab/daggers.