I got Invoker by randoming once, and I did much better then I thought I would (playing him as an initiator/support). I could definitely feel that with a few more games under my belt I'd be comfortable switching between all ten spells, the problem is that I'm worried about dragging down the rest of the team while I'm learning...
The thing about Invoker is, if you ever start messing up your spells, just try to calm down and stick to the stand bys until you do. You're better off right clicking and using a spell or two than messing up your spells and using the wrong ones.
I have tried to stay with few spells but i kept getting the wrong ones. The some dude kept asking me for red fire spells lol