@Xenonic: No noob questions here. We all started at level 1 in Cairn knowing nothing about the game too. And not saying that I have all the perfect answers, but that’s what this forum is all about! 
Some skills just really should be maxed out (or high as possible) because of their usefulness. Inquisitor’s Seal would be one of those top candidates in my book.
Other skills have diminishing returns the higher you level them up. Just for one example, the % chance of Execution (Nightblade) activating increases 3% from level 1 to 2… but from level 6 to 7, that increase has dwindled down to 0%, and only 1% from level 7 to 8. So you have to spend 2 points just for that extra 1% chance to activate. Sure, the other stats on Execution do consistently increase per level, but those 2 points might possibly be better spent else where.
Staying on Nightblade, let’s say you have gear that gives +1 to all Nightblade Skills and another item that gives +1 to Circle of Slaughter. So with those 2 extra points, you add 1 to unlock Ring of Steel (which automatically gets bumped to level 2) and the other point to unlock Circle of Slaughter (which gets bumped to level 3). Suddenly, those 2 points effectively becomes 5 points, giving your build the ability to stun enemies and a 20% chance for them to fumble attacks too! That’s lifesaving when a mob swarms you! The better “cost for the performance” is pretty clear.
However, aside from all the math equations and theorycrafting, it really depends on what you want to do with your character. The amulet that I am currently using in the Purifier build above is probably not the best in slot possible, but I really wanted the Apocalypse skill! Same goes for spending skill points. It’s all a balancing act.
Sorry for the wall of text. Hope it helps! 