What categories should I use my perks in?

I like 1h, archery, smithing (to upgrade weapons) sneak, pickpocket, illusion and possibly light armor if you find yourself getting detected alot. I dont think lockpicking is really worth it if you are any good at the mini-game.

It depends on what kind of assassin/thief you are.

Do you want to merely maximize your backstab damage?
Do you want to roleplay a thief that breaks into houses and steals things from people?
Do you use daggers or bow, or both?

Here are the main combat skills.
Sneak (core skill)
Smithing (for powerful daggers)
One-handed (for even more powerful dagger damage)
Archery (for situations where you cannot use daggers, such as dragons possibly)

The one with 15x damage with daggers might help a bit...