I like to role-play my characters so I don't make them as powerful as possible I just add perks to where it would fit my character. I play on Master and I've never used Smithing, Enchanting or Alchemy (I will aventually I think). I just use loot I find. The funny thing is that I have only put perks into Stealth and Archery at the moment. I've got like 11 perks to spend but I can't decide where first. So it's possible to play an Assassin for role-play on Master like I have.
If I wanted to make an assassin that's super powerful, I'd add Illusion for invisibility and Alchemy for Poisons. The thing with me is I keep my Thief/Mage/Warrior seperate. So no magic for me.
Similar to the way I play, my character is a smooth-talking thief/archer-assassin rather than the silent backstabby type, so I perked out Speech and the main branch of Lockpicking and took a couple of ranks of the first Pickpocket perk because it fits the character to be good at those things even if they are kind of useless in combat situations, and maxed out Stealth fairly early on simply because I was doing a lot of it (the top perk in the tree I have found to be about as useful as a chocolate teapot however, I wouldn't recommend it since it rarely seems to work as advertised!).
Now at around level 50 (really liking this character and have played just this one since release which is unusual for me!) and have also got some perks in Archery and Smithing, combat was difficult at the earlier levels because I was concentrating more on the role-playing skills than the combat ones, I haven't taken all the perks in archery, but I did take all ranks of overdraw and one rank of the one that slows time when drawing a bow. I haven't gone mad with the Smithing either to exploit it, just taken it naturally and done a little bit each level to make myself level-appropriate gear. As armor isn't so important to my character (the focus is on not getting hit in the first place, rather than being protected when she is hit!) I took the heavy perks because I was interested in creating better weapons, and just created and improved my very first Daedric bow which is fairly cool (
and Smithing isn't broken if you don't abuse it, people who say it is need to learn some self-control).
My skill level in both Alchemy and Enchanting is fairly good, but I haven't put any perks into those trees, if I did it would probably be overkill - I can get by with the weaker poisons and potions that I either craft myself or find as loot, same with enchanting I don't really need it beyond the ability to put one basic enchantment on a bow (I have 2 self made bows and the Nightingale bow which I switch between depending upon the combat situation, and dual wield Chillrend and Nightingale Blade if anything rarely manages to close in on me). At level 50 I can now one-shot giants from stealth but the dragons I am seeing at this level are still tricky because they usually detect me so I don't get the 3x stealth bonus, and it still takes me a while to defeat one.
I haven't really tried out much magic, not even Illusion, with this character but I will get around to it with others I am sure.